字符串等于和 == 与字符串连接[重复]

我试图理解字符串连接与字符串比较的输出。需要明确的是，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() 的输出是 concats，但是 == 的输出是does not连接。使用java的装箱功能，将布尔值与字符串连接起来。据我所知， equals 和 == 都返回布尔值。那么为什么会出现这种差异呢？谁能解释一下吗？

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();
    }
}

output

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

更新：回答

如果 s1==s2 没有括号，JVM 会将字符串比较为“Object andliteral Compare by double equal to :: jai”==“jai”，结果为false。所以sysout中的实际内容没有被打印。添加括号后，JVM 会将字符串比较为“jai”==“jai”，结果为

Object and literal compare by double equal to :: true

当你这样做时

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

你首先连接字符串"Object and literal compare by double equal to :: "用字符串s1，这将给出

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

然后，您正在检查该字符串是否是同一个对象（相同的引用）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。 “操作”运算符前置的一种方法是使用括号。首先解析括号内的运算：

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