Java Final变量

Java中的final变量只能被赋值一次，我们可以在声明中或之后赋值。

final int i = 10;
i = 30; // Error because i is final.

Java中的 final 变量是一个在声明时没有初始化的final变量。下面是一个关于空白final的简单例子。

// A simple blank final example 
final int i;
i = 30;

如何向对象的空白最终成员赋值？ 必须在构造函数中赋值。

// A sample Java program to demonstrate use and
// working of blank final
class Test {
    // We can initialize here, but if we initialize here,
    // then all objects get the same value.  So we use blank
    // final
    final int i;
 
    Test(int x)
    {
        // Since we have already declared i as final above,
        // we must initialize i in constructor. If we remove
        // this line, we get compiler error.
        i = x;
    }
}
 
// Driver Code
class Main {
    public static void main(String args[])
    {
        Test t1 = new Test(10);
        System.out.println(t1.i);
 
        Test t2 = new Test(20);
        System.out.println(t2.i);
    }
}

输出

10
20

如果我们在类中有多个构造函数或重载构造函数，那么空白最终变量必须在所有的构造函数中被初始化。然而，构造函数链可以用来初始化空白最终变量。

// A Java program to demonstrate that we can
// use constructor chaining to initialize
// final members
class Test
{
    final public int i;
 
    Test(int val)    {  this.i = val;  }
 
    Test()
    {
        // Calling Test(int val)
        this(10);
    }
 
    public static void main(String[] args)
    {
        Test t1 = new Test();
        System.out.println(t1.i);
 
        Test t2 = new Test(20);
        System.out.println(t2.i);
    }
}

输出