包装类习题和知识

包装类作用主要是包装基本数据类型，包装为对象数据类型

例：实现大小写字母的相互转换

package *;
class A{
	private char ch;

	public A(){}
	public A(char ch){this.ch=ch;}
	public char getCh(){return this.ch;}
	public char daXie(){
		if (ch>='A' && ch<='Z') {
		　　return (char)(ch+32);//ASCII是Unicode的子集
		}
		return ch;
	}
	public char xiaoXie(){
		if (ch>='a' && ch<='z') {
			return (char)(ch-32);
		}
		return ch;
	}
}
public class San {
	public static void main(String[] args) {
		char ch='r';
		A a=new A(ch);
		char c=a.getCh();
		System.out.println(c);
	}
}

　　定义一个抽象类，实现圆形，矩形和三角形的面积计算。

package *;
abstract class TuXing{
	public abstract double getMianJi();
}
class Yuan extends TuXing{
	private double r;
	public Yuan(){};
	public Yuan(double r){this.r=r;}
	public double getMianJi() {
		return Math.PI*r*r;
	}
}
class JuXing extends TuXing{
	private double x;
	private double y;
	public JuXing(){}
	public JuXing(double x, double y){
	　　this.x=x;this.y=y;
	}
	public double getMianJi() {
		return x*y;
	}
}
class Sanjiaoxing extends TuXing{
	private double a;
	private double b;
	private double c;
	public Sanjiaoxing(){}
	public Sanjiaoxing(double a,double b,double c){
		this.a=a;this.b=b;this.c=c;
	}
	public double getMianJi() {
		double p=(this.a+this.b+this.c)*0.5;
		double d=p*(p-this.a)*(p-this.b)*(p-this.c);
		double mianji=Math.sqrt(d);
		return mianji;
	}
}
public class Wu {
	public static void main(String[] args) {
		System.out.println(method(new Yuan(2)));
		System.out.println(method(new JuXing(2,3)));
		System.out.println(method(new Sanjiaoxing(3,4,5)));
	}
	public static double method(TuXing tuXing) {
		return tuXing.getMianJi();
	}
}