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The contract for that method states that when two objects are equal, their hash values must also be the same. That's why one must almost always override "),a("code",[t._v("hashCode()")]),t._v(" and "),a("code",[t._v("equals()")]),t._v(" together.")]),t._v(" "),a("p",[t._v("Pay special attention to the argument type of the "),a("code",[t._v("equals")]),t._v(" method. It is "),a("code",[t._v("Object obj")]),t._v(", not "),a("code",[t._v("Foo obj")]),t._v(". If you put the latter in your method, that is not an override of the "),a("code",[t._v("equals")]),t._v(" method.")]),t._v(" "),a("p",[t._v("When writing your own class, you will have to write similar logic when overriding "),a("code",[t._v("equals()")]),t._v(" and "),a("code",[t._v("hashCode()")]),t._v(". Most IDEs can automatically generate this for you.")]),t._v(" "),a("p",[t._v("An example of an "),a("code",[t._v("equals()")]),t._v(" implementation can be found in the "),a("code",[t._v("String")]),t._v(" class, which is part of the core Java API. Rather than comparing pointers, the "),a("code",[t._v("String")]),t._v(" class compares the content of the "),a("code",[t._v("String")]),t._v(".")]),t._v(" "),a("p",[t._v("Java 1.7 introduced the "),a("code",[t._v("java.util.Objects")]),t._v(" class which provides a convenience method, "),a("code",[t._v("equals")]),t._v(", that compares two potentially "),a("code",[t._v("null")]),t._v(" references, so it can be used to simplify implementations of the "),a("code",[t._v("equals")]),t._v(" method.")]),t._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 annotation punctuation"}},[t._v("@Override")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("boolean")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token 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However, that can fail in a few special cases which may not be obvious.  For example, some frameworks generate dynamic proxies of classes and these dynamic proxies are actually a different class.  Here is an example using JPA.")]),t._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"}},[t._v("Foo")]),t._v(" detachedInstance "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo")]),t._v(" mergedInstance "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" entityManager"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("merge")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("detachedInstance"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("mergedInstance"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("detachedInstance"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//Can never get here if equality is tested with getClass()")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//as mergedInstance is a proxy (subclass) of Foo")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("One mechanism to work around that limitation is to compare classes using "),a("code",[t._v("instanceof")])]),t._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 annotation punctuation"}},[t._v("@Override")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("public")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("final")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("boolean")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Object")]),t._v(" obj"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("!")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("obj "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("instanceof")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token boolean"}},[t._v("false")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//...compare fields")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("However, there are a few pitfalls that must be avoided when using "),a("code",[t._v("instanceof")]),t._v(".  Since Foo could potentially have other subclasses and those subclasses might override "),a("code",[t._v("equals()")]),t._v(" you could get into a case where a "),a("code",[t._v("Foo")]),t._v(" is equal to a "),a("code",[t._v("FooSubclass")]),t._v(" but the "),a("code",[t._v("FooSubclass")]),t._v(" is not equal to "),a("code",[t._v("Foo")]),t._v(".")]),t._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"}},[t._v("Foo")]),t._v(" foo "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("Foo")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("7")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("FooSubclass")]),t._v(" fooSubclass "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("new")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("FooSubclass")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("7")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token boolean"}},[t._v("false")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\nfoo"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("fooSubclass"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//true")]),t._v("\nfooSubclass"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("equals")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("foo"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//false")]),t._v("\n\n")])])]),a("p",[t._v("This violates the properties of symmetry and transitivity and thus is an invalid implementation of the "),a("code",[t._v("equals()")]),t._v(" method.  As a result, when using "),a("code",[t._v("instanceof")]),t._v(", a good practice is to make the "),a("code",[t._v("equals()")]),t._v(" method "),a("code",[t._v("final")]),t._v(" (as in the above example).  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All references using the fd have\n         * become unreachable. We can call close()\n         */")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("close")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n")])])]),a("p",[t._v("In this case it's the last chance to close the resource if that resource has not been closed before.")]),t._v(" "),a("p",[t._v("Generally it's considered bad practice to use "),a("code",[t._v("finalize()")]),t._v(" method in applications of any kind and should be avoided.")]),t._v(" "),a("p",[t._v("Finalizers are "),a("strong",[t._v("not")]),t._v(" meant for freeing resources (e.g., closing files).  The garbage collector gets called when (if!) the system runs low on heap space.  You can't rely on it to be called when the system is running low on file handles or, for any other reason.")]),t._v(" "),a("p",[t._v("The intended use-case for finalizers is for an object that is about to be reclaimed to notify some other object about its impending doom.  A better mechanism now exists for that purpose---the "),a("code",[t._v("java.lang.ref.WeakReference<T>")]),t._v(" class.  If you think you need write a "),a("code",[t._v("finalize()")]),t._v(" method, then you should look into whether you can solve the same problem using "),a("code",[t._v("WeakReference")]),t._v(" instead.  If that won't solve your problem, then you may need to re-think your design on a deeper level.")]),t._v(" "),a("p",[t._v("For further reading "),a("a",{attrs:{href:"http://www.informit.com/articles/article.aspx?p=1216151&seqNum=7",target:"_blank",rel:"noopener noreferrer"}},[t._v("here"),a("OutboundLink")],1),t._v(" is an Item about "),a("code",[t._v("finalize()")]),t._v(' method from "Effective Java" book by Joshua Bloch.')]),t._v(" "),a("h2",{attrs:{id:"object-constructor"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#object-constructor"}},[t._v("#")]),t._v(" Object constructor")]),t._v(" "),a("p",[t._v("All constructors in Java must make a call to the "),a("code",[t._v("Object")]),t._v(" constructor. This is done with the call "),a("code",[t._v("super()")]),t._v(". This has to be the first line in a constructor. 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This is used to reduce code duplication where multiple constructors need to do the same work.")]),t._v(" "),a("blockquote"),t._v(" "),a("p",[t._v("How do I call a specific constructor?")]),t._v(" "),a("p",[t._v("Given the example above, one can either call "),a("code",[t._v('new MyClass("argument")')]),t._v(" or "),a("code",[t._v('new MyClass("argument", 0)')]),t._v(". In other words, much like "),a("a",{attrs:{href:"http://stackoverflow.com/documentation/java/980/polymorphism/3187/method-overloading#t=201608202249467776787",target:"_blank",rel:"noopener noreferrer"}},[t._v("method overloading"),a("OutboundLink")],1),t._v(", you just call the constructor with the parameters that are necessary for your chosen constructor.")]),t._v(" "),a("blockquote"),t._v(" "),a("p",[t._v("What will happen in the Object class constructor?")]),t._v(" "),a("p",[t._v("Nothing more than would happen in a sub-class that has a default empty constructor (minus the call to "),a("code",[t._v("super()")]),t._v(").")]),t._v(" "),a("p",[t._v("The default empty constructor can be explicitly defined but if not the compiler will put it in for you as long as no other constructors are already defined.")]),t._v(" "),a("blockquote"),t._v(" "),a("p",[t._v("How is an Object then created from the constructor in Object?")]),t._v(" "),a("p",[t._v("The actual creation of objects is down to the JVM. Every constructor in Java appears as a special method named "),a("code",[t._v("<init>")]),t._v(" which is responsible for instance initializing. This "),a("code",[t._v("<init>")]),t._v(" method is supplied by the compiler and because "),a("code",[t._v("<init>")]),t._v(" is not a valid identifier in Java, it cannot be used directly in the language.")]),t._v(" "),a("blockquote"),t._v(" "),a("p",[t._v("How does the JVM invoke this "),a("code",[t._v("<init>")]),t._v(" method?")]),t._v(" "),a("p",[t._v("The JVM will invoke the "),a("code",[t._v("<init>")]),t._v(" method using the "),a("code",[t._v("invokespecial")]),t._v(" instruction and can only be invoked on uninitialized class instances.")]),t._v(" "),a("p",[t._v("For more information take a look at the JVM specification and the Java Language Specification:")]),t._v(" "),a("ul",[a("li",[t._v("Special Methods (JVM) - "),a("a",{attrs:{href:"https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("JVMS - 2.9"),a("OutboundLink")],1)]),t._v(" "),a("li",[t._v("Constructors - "),a("a",{attrs:{href:"https://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("JLS - 8.8"),a("OutboundLink")],1)])]),t._v(" "),a("h4",{attrs:{id:"syntax"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#syntax"}},[t._v("#")]),t._v(" Syntax")]),t._v(" "),a("ul",[a("li",[t._v("public final native Class<?> getClass()")]),t._v(" "),a("li",[t._v("public final native void notify()")]),t._v(" "),a("li",[t._v("public final native void notifyAll()")]),t._v(" "),a("li",[t._v("public final native void wait(long timeout) throws InterruptedException")]),t._v(" "),a("li",[t._v("public final void wait() throws InterruptedException")]),t._v(" "),a("li",[t._v("public final void wait(long timeout, int nanos)  throws InterruptedException")]),t._v(" "),a("li",[t._v("public native int hashCode()")]),t._v(" "),a("li",[t._v("public boolean equals(Object obj)")]),t._v(" "),a("li",[t._v("public String toString()")]),t._v(" "),a("li",[t._v("protected native Object clone() throws CloneNotSupportedException")]),t._v(" "),a("li",[t._v("protected void finalize() throws Throwable")])])])}),[],!1,null,null,null);s.default=e.exports}}]);