java - 两个字符串引用引用相同的字符串，但比较时，结果返回 false。

Question

s2和s3的值为“ab”，该字符串存储在字符串池中。 根据文档 s2==s3 应该返回 true，但它返回 false。 为什么？

 public class Sample {
      public static void main(String[] args) {
      String s="a";
      String s1="b";
      String s2=s+s1;
      String s3="ab";
      System.out.println(s2==s3);
   }
}

Answer 1

根据Java语言规范§15.18.1

15.18.1 String Concatenation Operator +

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

The result of string concatenation is a reference to a String object that is the concatenation of the two operand strings. The characters of the left-hand operand precede the characters of the right-hand operand in the newly created string.

The String object is newly created (§12.5) unless the expression is a constant expression (§15.28).

+ 将始终生成一个新字符串，除非表达式是常量。常量表达式的一个示例是"a"+ "b"。您的表达式不是常量，因为它包含非最终变量。

第 12.5 节对此进行了更明确的说明:

Execution of a string concatenation operator + (§15.18.1) that is not part of a constant expression (§15.28) always creates a new String object to represent the result.