字符串文字和字符串对象之间的区别 [英] Difference between addition of String Literals and String objects

问题描述

添加字符串文字和字符串对象有什么区别?

What is the difference between an addition of String Literal and String Object?

例如

    String s1 ="hello";
    String s2 ="hello1";
    String s3 ="hello" + "hello1";
    String s4 ="hellohello1";
    String s5 = s1 + s2;

    System.out.println(s3 == s4); // returns true
    System.out.println(s3 == s5); // return false
    System.out.println(s4 == s5); // return false

为什么s3/s4不能指向与s5相同的位置?

Why do s3/s4 not point to the same location as s5?

推荐答案

由于s1 + s2不是常量表达式，因为s1和s2不是final，因此其结果没有被求和，即创建了另一个对象来表示它，因此引用比较会生成false.

Because s1 + s2 is not a constant expression, since s1 and s2 are not final, therefore its result is not interned, i.e. another object is created to represent it, so reference comparison produces false.

JLS 3.10.5字符串文字 a>:

使用String.intern方法对字符串文字(或更一般地说，是常量表达式的值(第15.28节)的字符串)进行插入"，以便共享唯一的实例.

String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are "interned" so as to share unique instances, using the method String.intern.

JLS 15.28常量表达式:

编译时常量表达式是表示原始类型或String的值的表达式，该值不会突然完成，并且仅使用以下内容组成:

A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:

• ...
• 引用常量变量的简单名称(第4.12.4节).

JLS 4.12.4 定义了final变量.

如果将s1和s2声明为final，则s3 == s5将是true.

If you declare s1 and s2 as final, s3 == s5 would be true.

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