自考java怎么学习

2，求50以内的质数问题。public class Demo2{ public static void main(String[]args) { int i,j; for(i=2;i<=50;i++) { for(j=2;j<=i/2;j++) if(j%i==0)break; if(j>i/2) System.out.println(i+"是质数"); } }}3，排*问题[注意别把for后面加了;号]。import javax.swing.*;public class Demo15{ public static void main(String[]args) { int n,j,k,space; String result=(String)JOptionPane.showInputDialog(null,"请输入一个整数","输入对话框",JOptionPane.PLAIN_MESSAGE,null,null,null); n=Integer.parseInt(result); space=40; for(j=0;j<=n;j++,space-=2) { for(int i=0;i=0;j--,space+=2) { for(int i=0;i=KeyEvent.VK_A&&t<=KeyEvent.VK_Z) { text.append((char)t+" "); count++; if(count%10==0) text.append("\n"); } } public void keyTyped(KeyEvent e){} public void keyReleased(KeyEvent e){}}8，单击按钮显示单击的次数，import javax.swing.*;import java.awt.event.*;import java.applet.*;public class Demo8 extends Applet implements ActionListener{ int n=0; JButton button=new JButton("单击按钮可以显示点击的次数"); public void init() { setSize(800,600); button.addActionListener(this); button.setSize(34,3); add(button); } public void actionPerformed(ActionEvent e) { n++; button.setText(n+" "); }}9，画五角星。import java.awt.*;import javax.swing.*;class MyPanel extends JPanel{ public void paintComponent(Graphics g) { int r=100; int x1=100; int x2=(int)(r*(1-Math.cos((18*Math.PI)/180))); int x3=(int)(r*(1+Math.cos((18*Math.PI)/180))); int x4=(int)(r*(1-Math.cos((54*Math.PI)/180))); int x5=(int)(r*(1+Math.cos((54*Math.PI)/180))); int y1=0; int y2=(int)(r*(1-Math.sin((18*Math.PI)/180))); int y3=(int)(r*(1-Math.sin((18*Math.PI)/180))); int y4=(int)(r*(1+Math.sin((54*Math.PI)/180))); int y5=(int)(r*(1+Math.sin((54*Math.PI)/180))); g.setColor(Color.RED); g.drawOval(0,0,200,200); g.drawLine(x1,y1,x4,y4); g.drawLine(x1,y1,x5,y5); g.drawLine(x2,y2,x3,y3); g.drawLine(x2,y2,x5,y5); g.drawLine(x3,y3,x4,y4); }}public class Demo9 extends JFrame{ Demo9() { super("Demo9"); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); getContentPane().add(new MyPanel()); setSize(800,600); setVisible(true); } public static void main(String[]args) { new Demo9(); }}