在java中使用「final」關鍵字

Question

我遇到了final關鍵字，該關鍵字顯然鎖定在一個值中，因此以後無法更改。但是當我嘗試使用它時，變量x仍然被改變。我在哪裏錯了？

public class Class { 
    // instance variable 
    private int x; 

    public Class() { 
    // initialise instance variables 
    final int x = 123; 
    } 

    public void sampleMethod() { 
    // trying to change it using final again 
    final int x = 257; 
    System.out.println(x); 

    } 
    public void sampleMethod2() { 
    // trying to change it using without using final 
    x = 999; 
    System.out.println(x); 
    } 

} 

Answer 1

public class Class { 
    // instance variable 
    private int x; // <-- This variable is NOT final. 

    public Class() { 
    // initialise instance variables 
    final int x = 123; // <-- This is NOT the instance variable x, but rather hiding this.x with a method local variable called 'x' 
    } 

    public void sampleMethod() { 
    // trying to change it using final again 
    final int x = 257; // <-- This is NOT the instance variable x, but rather hiding this.x with a method local variable called 'x' 
    System.out.println(x); 
    System.out.println(this.x); // see! 

    } 
    public void sampleMethod2() { 
    // trying to change it using without using final 
    x = 999; // This changes this.x, but this.x IS NOT final. 
    System.out.println(x); 
    } 
} 

現在讓我們看看我們如何真正創建一個最終的變量：

public class ClassWithFinalInstanceVariable { 
    private final int x; 
    public ClassWithFinalInstanceVariable() { 
     this.x = 100; // We can set it here as it has not yet been set. 
    } 
    public void doesNotCompileIfUncommented() { 
     // this.x = 1; 
     // If the line above is uncommented then the application would no longer compile. 
    } 

    public void hidesXWithLocalX() { 
     int x = 1; 
     System.out.println(x); 
     System.out.println(this.x);  
    } 
} 

Answer 2

您chould更改您的代碼：

public class Class { 

    private final int x; 

    public Class() { 
     x = 123; 
    } 

    ... 
} 

Answer 3

，而不用聲明三次，你應該一次聲明它的。 逸岸sampleMethod2()不需要

public class Class { 
    // instance variable 
    private final int x; 

    public Class() { 
    // initialise instance variables 
    x = 123; 
    } 

    public void sampleMethod() { 
    // You will get error here 
    x = 257; 
    System.out.println(x); 

    } 

}