当所有的构造器都会把一个相同的值赋值给某个指定的实例域时，我们可以进行显示初始化。

public class Dog {    private String name = "goudan";    private int age;    private char sex;    public Dog(int age) {        this.age = age;    }    public Dog(int age, char sex) {        this.age = age;        this.sex = sex;    }}