字符串等于和==字符串连接 [英] String equals and == with String concatenation

问题描述

我试图通过String compare的输出来理解String连接。为了清楚起见，我有一个类使用==和equals来比较两个字符串。我试图将==和equals（）的输出连接到一个字符串。 equals（）concats的输出，但== 的输出不 concat。使用java的装箱功能，与字符串连接的布尔值将联系。根据我的知识，equals和==都返回boolean。那为什么会有这种差异呢？任何人都可以解释这个吗？

I am trying to understand the String concatenation with the output of String compare. To be clear, i have the class to compare two strings using == and equals. I am trying to concat the output of == and equals() to a string. The output of equals() concats, but the output of == does not concat. Using boxing feature of java, the boolean concatenated with string will contact. Both equals and == returns boolean to my knowledge. So why is this difference? Can anyone explain on this?

public class StringHandler {

    public void compareStrings() {
        String s1 = new String("jai");
        String s2 = "jai";
        String s3 = "jai";
        System.out.println("Object and literal compare by double equal to :: "
                + s1 == s2);
        System.out.println("Object and literal compare by equals :: "
                + s1.equals(s2));
        System.out
                .println("Literal comparing by double equal to :: " + s2 == s3);
        System.out.println("Literal comparing by equals :: " + s2.equals(s3));
    }

    public static void main(String[] args) {
        StringHandler sHandler = new StringHandler();
        sHandler.compareStrings();
    }
}

输出

false
Object and literal compare by equals :: true
false
Literal compareing by equals :: true

更新：回答

对于s1 == s2没有括号，JVM将字符串比较为Object and literal compare by double等于:: jai==jai，结果为 false 。因此，不打印sysout中的实际内容。当添加括号时，JVM将字符串比较为jai==jai，结果为

Without parenthesis for s1==s2, the JVM compares the string as "Object and literal compare by double equal to :: jai" == "jai" and the result is false. So the actual content in sysout is not printed. When parenthesis is added the JVM compares the string as "jai" == "jai" and the result is

对象和文字比较双倍等于to :: true

推荐答案

当你这样做时

System.out.println("Object and literal compare by double equal to :: "
            + s1 == s2);

你是第一个连接字符串对象和文字比较双倍等于::，字符串为 s1 ，这将给出

you are first concatenating the string "Object and literal compare by double equal to :: " with the string s1, which will give

"Object and literal compare by double equal to :: jai"

然后，你正在检查此字符串是否与 s2 相同的对象（相同的引用）：

then, you are checking if this string is the same object (same reference) than s2:

"Object and literal compare by double equal to :: jai" == "jai"

这将是 false （输出将是 false ）。

换句话说，这是因为运算符优先级。在操作之前操纵操作符的一种方法是使用括号。括号内的操作将首先被解析：

In other words, it's because operators precedence. One way to "manipulate" operators precedende is to use parentheses. The operations inside parentheses will be parsed first:

System.out.println("Object and literal compare by double equal to :: " + (s1 == s2));

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