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map.computeIfPresent("kelly", (k,v)->v+10); //{john=20, paul=30, peter=40} //kelly not present\n    map.computeIfPresent("peter", (k,v)->v+10); //{john=20, paul=30, peter=50} // peter present, so increase the value \n\n')])])]),s("ol",[s("li",[t._v("Using "),s("strong",[t._v("compute")])])]),t._v(" "),s("p",[t._v("This method replaces the value of a key by the newly computed value")]),t._v(" "),s("div",{staticClass:"language- extra-class"},[s("pre",{pre:!0,attrs:{class:"language-text"}},[s("code",[t._v('\n   Map<String, Integer> map = new HashMap<String, Integer>();\n    map.put("john", 20);\n    map.put("paul", 30);\n    map.put("peter", 40);\n    map.compute("peter", (k,v)->v+50); //{john=20, paul=30, peter=90} //Increase the value \n\n')])])]),s("ol",[s("li",[t._v("Using "),s("strong",[t._v("merge")])])]),t._v(" "),s("p",[t._v("Adds the key-value pair to the map, if key is not present or value for the key is null\nReplaces the value with the newly computed value, if the key is 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It can also be used in a non-static context by removing all occurences of "),s("code",[t._v("static")]),t._v(".")]),t._v(" "),s("p",[t._v("In addition to that most implementations support "),s("code",[t._v("putAll")]),t._v(", which can add all entries in one map to another like this:")]),t._v(" "),s("div",{staticClass:"language-java extra-class"},[s("pre",{pre:!0,attrs:{class:"language-java"}},[s("code",[t._v("another"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("putAll")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("one"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n\n")])])]),s("h2",{attrs:{id:"merging-combine-and-composing-maps"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#merging-combine-and-composing-maps"}},[t._v("#")]),t._v(" Merging, combine and composing Maps")]),t._v(" "),s("p",[t._v("Use "),s("code",[t._v("putAll")]),t._v(" to put every member of one map into another.  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A key and its value are sometimes called a "),s("strong",[t._v("key/value pair")]),t._v(" or an "),s("strong",[t._v("entry")]),t._v(". Maps typically provide these features:")]),t._v(" "),s("ul",[s("li",[t._v("Data is stored into the map in key/value pairs.")]),t._v(" "),s("li",[t._v("The map may contain only one entry for a particular key. If a map contains an entry with a particular key, and you try to store a second entry with the same key, then the second entry will replace the first. In other words, this will change the value associated with the key.")]),t._v(" "),s("li",[t._v("Maps provide fast operations to test whether a key exists in the map, to fetch the value associated with a key, and to remove a key/value pair.")])]),t._v(" "),s("p",[t._v("The most commonly used map implementation is "),s("a",{attrs:{href:"https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html",target:"_blank",rel:"noopener noreferrer"}},[t._v("HashMap"),s("OutboundLink")],1),t._v(". It works well with keys that are strings or numbers.")]),t._v(" "),s("p",[t._v("Plain maps such as HashMap are unordered. Iterating through key/value pairs may return individual entries in any order. If you need to iterate through map entries in a controlled fashion, you should look at the following:")]),t._v(" "),s("li",[t._v("\n[Sorted maps](https://docs.oracle.com/javase/8/docs/api/java/util/SortedMap.html) such as [TreeMap](https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html) will iterate through keys in their natural order (or in an order that you can specify, by providing a [Comparator](https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html)). 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