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than one type parameter in a generic class or interface. Multiple type parameters can be used in a class or interface by placing a "),a("strong",[s._v("comma-separated list")]),s._v(" of types between the angle brackets. Example:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MultiGenericParam")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("S")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("private")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" 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fruitHelper.eatAll(grannySmithApples);//Still allowed, GrannySmith is a Fruit\n\n    Collection<Object> objects = new ArrayList<>();\n    fruitHelper.addApple(objects); // Allowed, as Object super Apple\n    objects.add(new Shoe()); // Not a fruit\n    objects.add(new IPhone()); // Not a fruit\n    //fruitHelper.eatAll(objects); // Compile error: may contain a Shoe, too!\n}\n\n')])])]),a("p",[s._v("Choosing the right "),a("code",[s._v("T")]),s._v(", "),a("code",[s._v("? super T")]),s._v(" or "),a("code",[s._v("? extends T")]),s._v(" is "),a("strong",[s._v("necessary")]),s._v(" to allow the use with subtypes. 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Note that the Java language doesn't support omitting the class or object on which the method is called ("),a("code",[s._v("this")]),s._v(" in the above example) when type arguments are explicitly provided.")]),s._v(" "),a("h2",{attrs:{id:"requiring-multiple-upper-bounds-extends-a-b"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#requiring-multiple-upper-bounds-extends-a-b"}},[s._v("#")]),s._v(' Requiring multiple upper bounds ("extends A & B")')]),s._v(" "),a("p",[s._v("You can require a generic type to extend multiple upper bounds.")]),s._v(" "),a("p",[s._v("Example: we want to sort a list of numbers but "),a("code",[s._v("Number")]),s._v(" doesn't implement "),a("code",[s._v("Comparable")]),s._v(".")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("extends")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Number")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("&")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Comparable")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("sortNumbers")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("List")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" n "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Collections")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("sort")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v(" n "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("In this example "),a("code",[s._v("T")]),s._v(" must extend "),a("code",[s._v("Number")]),s._v(" "),a("strong",[s._v("and")]),s._v(" implement "),a("code",[s._v("Comparable<T>")]),s._v(' which should fit all "normal" built-in number implementations like '),a("code",[s._v("Integer")]),s._v(" or "),a("code",[s._v("BigDecimal")]),s._v(" but doesn't fit the more exotic ones like "),a("code",[s._v("Striped64")]),s._v(".")]),s._v(" "),a("p",[s._v("Since multiple inheritance is not allowed, you can use at most one class as a bound and it must be the first listed. For example, "),a("code",[s._v("<T extends Comparable<T> & Number>")]),s._v(" is not allowed because Comparable is an interface, and not a class.")]),s._v(" "),a("h2",{attrs:{id:"benefits-of-generic-class-and-interface"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#benefits-of-generic-class-and-interface"}},[s._v("#")]),s._v(" Benefits of Generic class and interface")]),s._v(" "),a("p",[s._v("Code that uses generics has many benefits over non-generic code. Below are the main benefits")]),s._v(" "),a("h3",{attrs:{id:"stronger-type-checks-at-compile-time"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#stronger-type-checks-at-compile-time"}},[s._v("#")]),s._v(" Stronger type checks at compile time")]),s._v(" "),a("p",[s._v("A Java compiler applies strong type checking to generic code and issues errors if the code violates type safety. 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"),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Method of Closeable called on T and works fine too.")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("h3",{attrs:{id:"note"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#note"}},[s._v("#")]),s._v(" Note:")]),s._v(" "),a("p",[s._v("You cannot bind the generic parameter to either of the type using "),a("strong",[s._v("OR")]),s._v(" ("),a("code",[s._v("|")]),s._v(") clause. Only the "),a("strong",[s._v("AND")]),s._v(" ("),a("code",[s._v("&")]),s._v(") clause is supported.\nGeneric type can extends only one class and many interfaces. Class must be\nplaced at the beginning of the list.")]),s._v(" "),a("h2",{attrs:{id:"using-generics-to-auto-cast"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#using-generics-to-auto-cast"}},[s._v("#")]),s._v(" Using Generics to auto-cast")]),s._v(" "),a("p",[s._v("With generics, it's possible to return whatever the caller expects:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("private")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Map")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" data"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),s._v(" key"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" data"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("key"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("The method will compile with a warning. The code is actually more safe than it looks because the Java runtime will do a cast when you use it:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Bar")]),s._v(" bar "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" foo"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[s._v('"bar"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n")])])]),a("p",[s._v("It's less safe when you use generic types:")]),s._v(" "),a("div",{staticClass:"language-java 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(i.e. returning "),a("code",[s._v("List<String>")]),s._v(" would not trigger a "),a("code",[s._v("ClassCastException")]),s._v("; you'd eventually get it when taking elements out of the list).")]),s._v(" "),a("p",[s._v("To work around this problem, you can create an API which uses typed keys:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("final")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("static")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Key")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("List")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token 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the compiler compiles the "),a("strong",[s._v("Java source")]),s._v(" into "),a("strong",[s._v("Java bytecode")]),s._v(" it applies a process known as "),a("strong",[s._v("type erasure")]),s._v(", which converts all generic code into non-generic code, making impossible to distinguish among T types at runtime. The type used with "),a("code",[s._v("instanceof")]),s._v(" has to be "),a("strong",[a("a",{attrs:{href:"https://docs.oracle.com/javase/tutorial/java/generics/nonReifiableVarargsType.html",target:"_blank",rel:"noopener noreferrer"}},[s._v("reifiable"),a("OutboundLink")],1)]),s._v(", which means that all information about the type has to be available at runtime, and this is usually not the case for generic types.")]),s._v(" "),a("p",[s._v("The following class represents what two different classes of "),a("code",[s._v("Example")]),s._v(", "),a("code",[s._v("Example<String>")]),s._v(" and "),a("code",[s._v("Example<Number>")]),s._v(", look like after generics has stripped off by "),a("strong",[s._v("type erasure")]),s._v(":")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Example")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// formal parameter is gone")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("boolean")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("isTypeAString")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),s._v(" s"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" s "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("instanceof")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token 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punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// prints false")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("isAList")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("ArrayList")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// prints true")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("isAList")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("ArrayList")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Float")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// prints true")]),s._v("\n\n")])])]),a("p",[s._v("In fact, unbounded wildcard is considered a reifiable type.")]),s._v(" "),a("p",[a("strong",[s._v("Using a generic instance with instanceof")])]),s._v(" "),a("p",[s._v("The other side of the coin is that using an instance "),a("code",[s._v("t")]),s._v(" of "),a("code",[s._v("T")]),s._v(" with "),a("code",[s._v("instanceof")]),s._v(" is legal, as shown in the following example:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Example")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("boolean")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("isTypeAString")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" t "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("instanceof")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// No compilation error this time")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("because after the type erasure the class will look like the following:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Example")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// formal parameter is gone")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("boolean")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("isTypeAString")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" t "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("instanceof")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// No compilation error this time")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("Since, even if the type erasure happen anyway, now the JVM can distinguish among different types in memory, even if they use the same reference type ("),a("code",[s._v("Object")]),s._v("), as the following snippet shows:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),s._v(" obj1 "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[s._v('"foo"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// reference type Object, object type String")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),s._v(" obj2 "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Integer")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[s._v("11")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// reference type Object, object type Integer")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("obj1 "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("instanceof")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// true")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("System")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("out"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("println")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("obj2 "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("instanceof")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// false, it's an Integer, not a String")]),s._v("\n\n")])])]),a("h2",{attrs:{id:"different-ways-for-implementing-a-generic-interface-or-extending-a-generic-class"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#different-ways-for-implementing-a-generic-interface-or-extending-a-generic-class"}},[s._v("#")]),s._v(" Different ways for implementing a Generic Interface (or extending a Generic Class)")]),s._v(" "),a("p",[s._v("Suppose the following generic interface has been declared:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("interface")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericInterface")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("Below are listed the possible ways to implement it.")]),s._v(" "),a("p",[a("strong",[s._v("Non-generic class implementation with a specific type")])]),s._v(" "),a("p",[s._v("Choose a specific type to replace the formal type parameter "),a("code",[s._v("<T>")]),s._v(" of "),a("code",[s._v("MyGenericClass")]),s._v(" and implement it, as the following example does:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("NonGenericClass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("implements")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericInterface")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// type T has been replaced by String")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("This class only deals with "),a("code",[s._v("String")]),s._v(", and this means that using "),a("code",[s._v("MyGenericInterface")]),s._v(" with different parameters (e.g. "),a("code",[s._v("Integer")]),s._v(", "),a("code",[s._v("Object")]),s._v(" etc.) won't compile, as the following snippet shows:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("NonGenericClass")]),s._v(" myClass "),a("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("NonGenericClass")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nmyClass"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token string"}},[s._v('"foo_string"')]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// OK, legal")]),s._v("\nmyClass"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[s._v("11")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// NOT OK, does not compile")]),s._v("\nmyClass"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// NOT OK, does not compile")]),s._v("\n\n")])])]),a("p",[a("strong",[s._v("Generic class implementation")])]),s._v(" "),a("p",[s._v("Declare another generic interface with the formal type parameter "),a("code",[s._v("<T>")]),s._v(" which implements "),a("code",[s._v("MyGenericInterface")]),s._v(", as follows:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericSubclass")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("implements")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericInterface")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("T")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// type T is still the same")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// other methods...")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("Note that a different formal type parameter may have been used, as follows:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericSubclass")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("U")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("implements")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericInterface")]),a("span",{pre:!0,attrs:{class:"token generics"}},[a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("<")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("U")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(">")])]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// equivalent to the previous declaration")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("U")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// other methods...")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[a("strong",[s._v("Raw type class implementation")])]),s._v(" "),a("p",[s._v("Declare a non-generic class which implements "),a("code",[s._v("MyGenericInteface")]),s._v(" as a "),a("strong",[s._v("raw type")]),s._v(" (not using generic at all), as follows:")]),s._v(" "),a("div",{staticClass:"language-java extra-class"},[a("pre",{pre:!0,attrs:{class:"language-java"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("class")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericSubclass")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("implements")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("MyGenericInterface")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("public")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Object")]),s._v(" t"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// type T has been replaced by Object")]),s._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// other possible methods")]),s._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n")])])]),a("p",[s._v("This way is "),a("strong",[s._v("not")]),s._v(" recommended, since it is not 100% safe at runtime because it mixes up "),a("strong",[s._v("raw type")]),s._v(" (of the subclass) with "),a("strong",[s._v("generics")]),s._v(" (of the interface) and it is also confusing. Modern Java compilers will raise a warning with this kind of implementation, nevertheless the code - for compatibility reasons with older JVM (1.4 or earlier) - will compile.")]),s._v(" "),a("p",[s._v("All the ways listed above are also allowed when using a generic class as a supertype instead of a generic interface.")]),s._v(" "),a("h4",{attrs:{id:"syntax"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#syntax"}},[s._v("#")]),s._v(" Syntax")]),s._v(" "),a("ul",[a("li",[s._v("class ArrayList"),a("E",[s._v(" {} // a generic class with type parameter E")])],1),s._v(" "),a("li",[s._v("class HashMap<K, V> {} // a generic class with two type parameters K and V")]),s._v(" "),a("li",[a("E",[s._v(" void print(E element) {} // a generic method with type parameter E\n")])],1),s._v(" "),a("li",[s._v("ArrayList"),a("String",[s._v(" names; // declaration of a generic class")])],1),s._v(" "),a("li",[s._v("ArrayList<?> objects; // declaration of a generic class with an unknown type parameter")]),s._v(" "),a("li",[s._v("new ArrayList"),a("String",[s._v("() // instantiation of a generic class")])],1),s._v(" "),a("li",[s._v('new ArrayList<>() // instantiation with type inference "diamond" (Java 7 or later)')])]),s._v(" "),a("h4",{attrs:{id:"remarks"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#remarks"}},[s._v("#")]),s._v(" Remarks")]),s._v(" "),a("p",[s._v("Generics are implemented in Java through Type erasure, which means that during runtime the Type information specified in the instantiation of a generic class is not available. For example, the statement "),a("code",[s._v("List<String> names = new ArrayList<>();")]),s._v(" produces a list object from which the element type "),a("code",[s._v("String")]),s._v(" cannot be recovered at runtime. However, if the list is stored in a field of type "),a("code",[s._v("List<String>")]),s._v(", or passed to a method/constructor parameter of this same type, or returned from a method of that return type, then the full type information "),a("strong",[s._v("can")]),s._v(" be recovered at runtime through the Java Reflection API.")]),s._v(" "),a("p",[s._v("This also means that when casting to a generic type (e.g.: "),a("code",[s._v("(List<String>) list")]),s._v("), the cast is an "),a("strong",[s._v("unchecked cast")]),s._v(". Because the parameter "),a("code",[s._v("<String>")]),s._v(" is erased, the JVM cannot check if a cast from a "),a("code",[s._v("List<?>")]),s._v(" to a "),a("code",[s._v("List<String>")]),s._v(" is correct; the JVM only sees a cast for "),a("code",[s._v("List")]),s._v(" to "),a("code",[s._v("List")]),s._v(" at runtime.")])])}),[],!1,null,null,null);t.default=e.exports}}]);