一、满足以下三个条件,一个final变量就不再是一个变量,而是一个直接量。

  1. 使用final修饰符修饰。
  2. 在申明的时候就进行初始化
  3. 初始化的值在编译器就可以确定。

二、在什么情况下初始化的值在编译期是可以确定下来的?

  被赋的表达式只是基本的算术表达式或字符串链接运算,没有访问普通变量,调用方法。  

package fianlFieldCase;

public class Test {

    public static void main(String[] args) {

        final String str1 = "final" + "field";
final String str2 = "java" + 8; //下面的str3的值因为调用了方法,所以值在编译期不能确定
final String str3 = "java" + String.valueOf(8);
final int a = 2 + 3;
final double b = 1.5 / 3;
System.out.println("java8" == str2);
System.out.println("java8" == str3);
}
}

  编译以后的class文件如下:

aaarticlea/png;base64,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" alt="" />

  从编译以后的结果可以看出,除了str3,其他变量的值在编译期间就可以确定下来的。由于str2是一个直接量,它直接被替换成"java8",str2便指向字符串常量池中的"java8",所以当"java8"和str2进行==比较的时候直接返回true。但是str3的值要在运行期才能确定,这样就不能指向常量池中的”java8“。

  常量池是专门用于管理在编译期被确定并被保存在已编译的class文件中的一些数据。它包括了关于类、方法、接口中的常量,还包括字符串常量。

  例如执行String a = "java",就会在字符串常量区缓存一个"java";当再执行String b = "java",b就直接指向刚刚缓存的"java"。所以a==b 返回true。

  

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