Question

关于Java的一个简单问题

我写作业的时候遇到了一些问题，其实挺简单的，可是我还是不懂，还希望大家帮帮忙啊。急用~~
编写两个类Point2D，Point3D来表示二维空间和三维空间的点，
（1）Point2D有两个整型成员变量x, y (分别为二维空间的X,Y方向坐标)，Point2D的构造方法要实现对其成员变量x, y的初始化。
（2）Point3D是Point2D的直接子类，它有有三个整型成员变量x,y,z (分别为三维空间的X,Y,Z方向坐标)，Point3D有两个构造方法：Point3D(int x,int y,int z)和Point3D(Point2D p,int z)，两者均可实现对Point3D的成员变量x, y,z的初始化。
Point3d（Point2d p，z）在方法体里面如果写p.，出现的下拉菜单里面并没有x，y，z以及在Point2d里面定义的那些函数啊?这是怎么回事啊？是不是说明不能这样用？

Answer 1

大致上这么写。 protected这个type意思是所有子类也可以用这个变量。

class Point2D
{
public Point2D(int x, int y){
this.x = x;
this.y = y;
}
public int getX()
{return this.x;}
public int getY()
{return this.y;}

protected int x;
protected int y;
}
class Point3D extends Point2D
{
public Point3D(int x,int y,int z)
{ super(x, y);
this.z = z;
}
Point3D(Point2D p,int z)
{ this.x = p.getX();
this.y = p.getY();
this.z = z;
}
protected int z;
}

Answer 2

public class Point2D{
int x;
int y;

public Point2D() {
super();
}

public Point2D(int x, int y) {
super();
this.x = x;
this.y = y;
}
}

=========================

public class Point3D extends Point2D {
int z;
Point2D p;

public Point3D(int x, int y, int z) {
super(x, y);
this.z = z;
}

public Point3D(Point2D p,int z){
this.x = p.x;
this.y = p.y;
this.z = z;
}
}

Answer 3

public class Point2D {
private int x, y;
public Point2D(int x, int y) {
this.x = x;
this.y = y;
}
public int getX() {return x;}
public int getY() {return y;}
}

public class Point3D extends Point2D {
private int z;
public Point3D(int x, int y, int z) {
this.x = x;
this.y = y;
this.z = z;
}
public Point3D(Point2D p, int z) {
this.x = p.getX();
this.y = p.getY();
this.z = z;
}
}

Answer 4

public class Point2D
{
private int x;
private int y;

protected Point2D()
{
}

public Point2D(int x,int y)
{
this.x = x;
this.y = y;
}

public int getX() {
return x;
}
public void setX(int x) {
this.x = x;
}
public int getY() {
return y;
}
public void setY(int y) {
this.y = y;
}
}

class Point3D extends Point2D
{
private int z;

private Point3D()
{
}

public Point3D(int x,int y,int z)
{
super(x,y);
this.z = z;
}

public Point3D(Point2D p,int z)
{
super(p.getX(),p.getY());
this.z = z;
}

public int getZ()
{
return z;
}

public void setZ(int z)
{
this.z = z;
}
}
那是因为父类的变量是private的，所以直接用.出来不了。改成public,protected就可以了，但是为了代码的安全性，变量都是使用的private。

Answer 5

import java.util.*;
public class Test{
public static void max_min(int a[]){
int b[]= new int [a.length];
int k=0 ,i,j;
for( i=0;i<a.length; i++)
b[i]=a[i];
for( j = 0;j<b.length-1; j++)
for( i = j,k = i ; i<b.length-1 ; i++)
{

if(b[i]<b[i+1])
k= i+1;
}
if(k!=i)
{
int t;
t=a[j];
a[j]=a[k];
a[k]= t;
//交换
}
for( i = 0; i<b.length; i++)
System.out.println(b[i]);
}
public static void main(String args[]){
int list[]= new int[3];
Random rand = new Random();
for(int i = 0; i< list.length; i++ )
{
list[i] = rand.nextInt()%100;
max_min(list);
System.out.println(list[i]);
}
}
}