 Circle 定義1個變數：半徑；定義2個方法：求面積、求周長。

 3個基本類都要定義相應的構造方法，對變數進行初始化。

解：建立三個類，分別表示三角形，梯形，以及圓形，三角形中定義方法求取周長，以及獲取三邊長，以此類推分別定義梯形和圓形。

程式碼如下：

package test3;

class Triangle{
	double  a,b,c;
	public Triangle()	{
		
	}
	public Triangle(double a, double b, double c){
		this.a=a;
		this.b=b;
		this.c=c;
	}
	
	public double C(){
		return a+b+c;
	}
}

class Ladder{
	double up,down,height;
	public Ladder(){
		
	}
	public Ladder(double up, double down, double height){
		this.up=up;
		this.down=down;
		this.height=height;
	}
	public double Area()
	{
		return (up+down)*height/2;
	}
}

class Circle{
	double radius;
	public Circle(){
		
	}
	public Circle(double radius){
		this.radius=radius;
	}
	
	public double Area(){
		return  Math.PI*radius*radius;
	}
	
	public double C(){
		return  Math.PI*radius*2.;
	}
}


public class test3_a{
	public static void main(String[] args) {
		Circle cc=new Circle(3);
		System.out.println(cc.Area());
		System.out.println(cc.C());
		
		Ladder  ld=new Ladder(1,2,3);
		System.out.println(ld.Area());
		
		Triangle tr=new Triangle(1,2,3);
		System.out.println(tr.C());
	}
}
 

（2）編寫一個賬戶類Account，它包括：一個名為id的int型賬號碼屬性，一個名為balance的double型的賬號餘額屬性，定義一個型別為java.util.Date的屬性dateCreated，用於記錄賬號的建立日期。同時，定義無參的建構函式，一個名為withDraw的方法從賬號提取特定數目的金額，一個名為deposit的方法向賬號存入特定數目的金額。請編寫測試程式，測試各個方法。

package test3;

import java.util.Calendar;
import java.util.Date;

class Account{
	int id;
	double balance;
	Date dateCreated;
	
	public Account(){
		
	}
	public Account(int id, double balance){
		this.id=id;
		this.balance=balance;
		this.dateCreated=(Date) Calendar.getInstance().getTime();
	}
	
	public boolean withDraw(double money){
		if(this.balance>money){
			this.balance-=money;
			return true;
		}
		return false;			
	}
	
	public  void deposit(double money){
		this.balance+=money;
	}
	public double getBalance(){
		return this.balance;
	}
	
}

public class test4_b {	
	public static void main(String[] args) {
		Account ac1=new Account(12,100.);
		boolean result =ac1.withDraw(1000);
		if(result)
			System.out.println("Success");
		else {
			System.out.println("Fail");
			
			ac1.deposit(1000);
			System.out.println(ac1.getBalance());
		}
	}
}<strong>
</strong>
 

package test3;

class Student{
	private  int  id;
	private  String  name;
	private char sex;
	private  int age;
	private double averageScore;
	
	public double getAverageScore(){
		return this.averageScore;
	}
	
	public void setAverageScore(double score){
		this.averageScore=score;
	}
	
	public Student(){
		
	}
	public Student(int id, String  name,  char sex, int age, double score){
		this.id=id;
		this.name=name;
		this.sex=sex;
		this.age=age;
		this.averageScore=score;
	}
	
	public void print()
	{
		System.out.println(id+" "+name+" "+sex+" "+age+" "+averageScore);
	}
}

public class test3_c {

	public static void main(String[] args) {
		Student  st1=new Student(1,"zhang",'f',23,90);
		st1.print();
		st1.setAverageScore(80);
		st1.print();
		
	}
}<strong>
</strong>