public class Person{
 private int age=0;
 private String name="noname";
 private char sex='M';
 public Person(String n,int a,char s){
         name=n;
         if(a>=0&&a<140) age=a;
         else age=0;
         if(s=='M') sex=s;
         else sex='F';
 }
 public void introduceme(){
     System.out.println("my name is:"+name+"\tmy age is :"+age);
     if(sex=='M') System.out.println("i an man!");
     else System.out.println("i am woman!");
 }
     public String getName(){return name;}
     public void setName(String n){name=n;}
     public int getAge(){return age;}
     public void setAge(int a){
         if(a>=0&&a<140) age=a;
         else age=0;
     }
     public char getSex(){return sex;}
     public void setSex(char s){
         if(s=='M') sex='M';
         else sex='F';
     }
     public boolean equals(Person a){
         if(this.name.equals(a.name)&&this.age==a.age&&this.sex==a.sex)
             return true;
         else
             return false;
     }
     public String toString(){
         return name+","+sex+","+age;
     }
}
 class PersonTest
 {
     public static void main(String args[]){
         Person p1,p2;
         p1=new Person("张三",28,'M');
         p2=new Person();
         p2.setName("陈红");
         p2.setAge(38);p2.setSex('F');
         p1.introduceme();
         p2.introduceme();
     }
 }

没有默认的构造函数。

谢谢 知道问题了 我是新手 以后还多多指教！