显示域初始化

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;
    }
}