字符串等于和==字符串连接

Question

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？谁能解释一下呢？

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，不会打印sysout中的实际内容。添加括号后，JVM将字符串比作“jai”==“jai”，结果为

Object and literal compare by double equal to :: true

Answer 1

当你这样做时

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

您首先将字符串"Object and literal compare by double equal to :: "与字符串s1连接起来，这将提供

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

然后，您正在检查此字符串是否与s2相同的对象（相同的引用）：

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

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：

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

Answer 2

<强>问题：

+ s2 == s3

+符号的优先级高于==，因此它会在执行append之前先执行（==），因此只会"false"

您需要放置括号，以便检查字符串比较的结果。

同样parenthesis的{​​{3}}高于+，它会在将字符串附加到字符串之前先对其进行比较。

<强>样品：

 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 compareing by double equal to :: " + (s2 == s3));
    System.out.println("Literal compareing by equals :: " + (s2.equals(s3)));

运算符优先级：

Answer 3

添加括号！

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

PLus和其他算术运算具有比比较运算更高的优先级。使用'+'进行连接不会改变它。

Answer 4

这是一个操作顺序问题。它被解释如下（括号添加）：

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 compareing by double equal to :: " + s2)  == s3);
System.out.println("Literal compareing by equals :: " + (s2.equals(s3)) );