public class Student { private static List<Student> studentList = new ArrayList<Student>(); static { studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); } private final static String S_NAME = "aaa"; // String pool. private final static String NAME = new String("bbb"); private final static Integer AGE = 11; private String sname; private String name; private Integer age; public Student(String sname, String name, Integer age) { this.sname = sname; this.name = name; this.age = age; } public static void main(String[] args) { System.out.println(studentList); } @Override public String toString() { return "Student [sname=" + sname + ", name=" + name + ", age=" + age + "]"; } }
下面是输出:
[Student [sname=aaa, name=null, age=null], Student [sname=aaa, name=null, age=null]]
按照以前的理解,不应该是这个结果的。
于是乎问了下大神,得出了以下结论:
1,静态变量声明和初始化是两个过程;
2, String字面常量有个pool, 每次有新的常量出现的时候, 先把它放入pool, 然后直接把变量引用指向pool中的位置;
3. java中的字面常量在编译的时候被替换成真实的值,具体表现为字符串常量和基本类型常量。
4.static 块java language specification是保证happen before的,也就是有顺序的。
所以代码等同于:
private static List<Student> studentList = null; private final static String S_NAME = "aaa"; // String pool. private final static String NAME = null; private final static Integer AGE = null; static { studentList = new ArrayList<Student>(); studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); NAME = new String("bbb"); AGE = 11; }
所以输出结果可定不对啊!!!
改成下面这样就可以了,其实就是换一下声明顺序。
private static List<Student> studentList = new ArrayList<Student>(); private final static String S_NAME = "aaa"; // String pool. private final static String NAME = new String("bbb"); private final static Integer AGE = 11; static { studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); studentList.add(new Student(Student.S_NAME, Student.NAME, Student.AGE)); }
输出结果
[Student [sname=aaa, name=bbb, age=11], Student [sname=aaa, name=bbb, age=11]]
自身引用自身的静态变量时才会发生上面个的情况( Student引用Student的static),
如果Student引用Teacher的静态常量就不会有上面这个问题,因为Teacher已经初始化完成所有的静态常量了。