Java String declaration [duplicate]

开发者_如何学PythonThis question already has answers here: What is the difference between "text" and new String("text")? (13 answers) Closed 2 years ago.

What is the difference between String str = new String("SOME") and String str="SOME" Does these declarations gives performance variation.

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String str = new String("SOME")

always create a new object on the heap

String str="SOME" 

uses the String pool

Try this small example:

        String s1 = new String("hello");
        String s2 = "hello";
        String s3 = "hello";

        System.err.println(s1 == s2);
        System.err.println(s2 == s3);

To avoid creating unnecesary objects on the heap use the second form.

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There is a small difference between both.

Second declaration assignates the reference associated to the constant SOMEto the variable str

First declaration creates a new String having for value the value of the constant SOME and assignates its reference to the variable str.

In the first case, a second String has been created having the same value that SOME which implies more inititialization time. As a consequence, you should avoid it. Furthermore, at compile time, all constants SOMEare transformed into the same instance, which uses far less memory.

As a consequence, always prefer second syntax.

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String s1 = "Welcome"; // Does not create a new instance  
String s2 = new String("Welcome"); // Creates two objects and one reference variable  

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First one will create new String object in heap and str will refer it. In addition literal will also be placed in String pool. It means 2 objects will be created and 1 reference variable.

Second option will create String literal in pool only and str will refer it. So only 1 Object will be created and 1 reference. This option will use the instance from String pool always rather than creating new one each time it is executed.