为什么这段代码输入字符串并输出int不起作用？Java的

Question

可能重复:
Java String.equals与==

我认为这将是一种结构化选择器方法的简洁方法,但输出不是前两个if语句而只输出最后一个

    public int myPickerMethod(){

        System.out.println("please select from the options ");
        System.out.println("please select 1 for option 1 ");
        System.out.println("please select 2 please select 2 for option 2");
        String input = keyboard.readLine();
        System.out.println("input = " + input);     

        if(input=="1"){

                return 1;
        }
        else if(input=="2"){
            return 2;
        }
        else{
            return 42;
        }
   }

这是终端的结果:

   please select from the options 
   please select 1 for option 1 
   please select 2 please select 2 for option 2
   1
   input = 1
   response = 42

如果我输入2,则同样如此."response"print语句是来自main类中print语句的方法的输出.

我以前没试过这种方式,但我认为它应该有效.我真的不明白为什么不是.有人能清除这个吗？谢谢

Answer 1

在Java中,您需要使用equals方法比较Strings:

if ( input.equals("1") ) {
    // do something...
}

从Java 7开始,您也可以在switch语句中使用Strings:

switch ( input ) {

    case "1":
        // do something...
        break;

    case "2":
        // do something...
        break;

}

编辑:补充我的答案,这里是一个类的例子,它使用带字符串的开关和类的反汇编代码(使用javap -c)及其工作原理(标签8和11).

Foo.java

public class Foo {

    public static void main( String[] args ) {

        String str = "foo";

        switch ( str ) {

            case "foo":
                System.out.println( "Foo!!!" );
                break;

            case "bar":
                System.out.println( "Bar!!!" );
                break;

            default:
                System.out.println( "Neither Foo nor Bar :(" );
                break;

        }

    }

}

反汇编的Foo.class代码:

Compiled from "Foo.java"
public class Foo {
  public Foo();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String foo
       2: astore_1      
       3: aload_1       
       4: astore_2      
       5: iconst_m1     
       6: istore_3      
       7: aload_2       
       8: invokevirtual #3                  // Method java/lang/String.hashCode:()I  << hashCode here! (I wrote this!)
      11: lookupswitch  { // 2

                 97299: 50                  // 97299: hashCode of "bar" (I wrote this!)

                101574: 36                  // 101574: hashCode of "foo" (I wrote this!) (yep, they where swaped. the lesser hashCode first)
               default: 61
          }
      36: aload_2       
      37: ldc           #2                  // String foo
      39: invokevirtual #4                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      42: ifeq          61
      45: iconst_0      
      46: istore_3      
      47: goto          61
      50: aload_2       
      51: ldc           #5                  // String bar
      53: invokevirtual #4                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
      56: ifeq          61
      59: iconst_1      
      60: istore_3      
      61: iload_3       
      62: lookupswitch  { // 2

                     0: 88

                     1: 99
               default: 110
          }
      88: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      91: ldc           #7                  // String Foo!!!
      93: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      96: goto          118
      99: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
     102: ldc           #9                  // String Bar!!!
     104: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
     107: goto          118
     110: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
     113: ldc           #10                 // String Neither Foo nor Bar :(
     115: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
     118: return        
}

一个有趣的事情是用整数创建另一个开关(标签62),它是做"实际工作"的.

Answer 2

1.用于在java中.equals进行比较Objects,并且String是Java中的Object.

例如: if(input.equals("1"))

2.这是可选的,但如果您尝试使用Scanner类,它也会很好.

例如:

Scanner scan = new Scanner(System.in);
String input = scan.nextLine();

3.它也将建议使用switch的语句来代替 if-else梯,如果开关用来使用String那么你必须有 jdk 1.7 or above,否则,如果你使用的是JDK版本低于 1.7,那么你就可以转换为字符串的字符,然后在交换机中使用它.

Answer 3

对于测试Object(非基本类型)的相等性,请使用Object.equals().

if(input.equals("1")) {

    return 1;
}

该==运营商检查是否引用的对象是相等的.参考相等性的测试String.equals() method在其他检查中进行.以下是该String.equals()方法的Java源代码:

public boolean equals(Object anObject) {
     if (this == anObject) {      // Reference equality
         return true;
     }
     if (anObject instanceof String) {
         String anotherString = (String)anObject;
         int n = count;
         if (n == anotherString.count) {  // Are the strings the same size?
             char v1[] = value;
             char v2[] = anotherString.value;
             int i = offset;
             int j = anotherString.offset;
             while (n-- != 0) {
                 if (v1[i++] != v2[j++])        // Compare each character
                     return false;
             }
             return true;
         }
     }
     return false;
}