为什么JDK 1.4和1.5之间的输出不同？ [英] Why the output is different between JDK 1.4 and 1.5?

问题描述

我正在使用JDK 1.4和1.5运行此代码并获得不同的结果。为什么会这样？

I'm running this code with JDK 1.4 and 1.5 and get different results. Why is it the case?

String str = "";
int test = 3;

str = String.valueOf(test);
System.out.println("str[" + str + "]\nequals result[" + (str == "3") + "]");

if (str == "3") {
    System.out.println("if");
} else {
    System.out.println("else");
}

输出：

• on jdk 1.4

• on jdk 1.4

str[3]
equals result[true]
if

on jdk 1.5

on jdk 1.5

str[3]
equals result[false]
else

推荐答案

根据此页面，整数#toString 方法（由 String＃valueOf（int））在1.4中实现如下：

According to this page, the Integer#toString method (which is called by String#valueOf(int)) is implemented like this in 1.4:

public static String toString(int i) {  
    switch(i) {  
        case Integer.MIN_VALUE: return "-2147483648";  
        case -3: return "-3";  
        case -2: return "-2";  
        case -1: return "-1";  
        case 0: return "0";  
        case 1: return "1";  
        case 2: return "2";  
        case 3: return "3";  
        case 4: return "4";  
        case 5: return "5";  
        case 6: return "6";  
        case 7: return "7";  
        case 8: return "8";  
        case 9: return "9";  
        case 10: return "10";  
    }  
    char[] buf = (char[])(perThreadBuffer.get());  
    int charPos = getChars(i, buf);  
    return new String(buf, charPos, 12 - charPos);  
}

这可以解释你的结果，因为字符串文字 3被实习，3==3总是返回true。

That would explain your result because the string literal "3" is interned and "3" == "3" always returns true.

您可以尝试10和11来验证这一点。

You can try with 10 and 11 to verify this.

注意：如前所述，整数#toString 没有说明返回的字符串是否会被实习，因此你问题中的两个输出同样有效。

Note: as already mentioned, the javadoc of Integer#toString does not say whether the returned string will be interned or not so both outputs in your question are equally valid.

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