primzahl.java

这是一个判断一个数是不是质数的小程序.就是在applet上输入一个数,输出会告诉你这个数是不是质数.

// Primzahl.java                                                   
//                                                                  
// input: natural number n
// output: smallest prime number p with p > n
//
// method: apply prime test to n+1, n+2, ... until prime is found,
//         prime testing of z is done by testing numbers k = 2 ...  with
//         k * k <= z whether they are factors of z
//

import java.awt.*;
import java.applet.Applet;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class Primzahl extends Applet {
        Label inputPrompt;        // declare Label
        TextField input, output;  // declare textfields for input and output
    
        int n,        // the input integer read from user
            z,        // candidate for the prime, set to n+1, n+2 etc.
            k;        // possible divisor of z
        boolean  divisorFound;     // Boolean, indicates that a divisor 
                                   // k of z has been found
    
        // setup the graphical user interface components
        // and initialize labels and text fields
        public void init() {
                // set layout
                setLayout(new FlowLayout( FlowLayout.LEFT ));
                // set Font
                Font myFont = new Font("Times", Font.PLAIN, 18);
                setFont( myFont );
                setSize( 800, 100 );
        
                inputPrompt = new Label("Geben Sie die Zahl n ein "
                        + "und dr\u00fccken Sie Return.");
                input = new TextField( 10 );
                input.addActionListener(new ActionListener() {
                        public void actionPerformed(ActionEvent e) {
                                // call method for finding next prime
                                findPrime();
                        }
                });    // action will be on input field

                // output will be text in a field
                output = new TextField(60);
                output.setEditable(false);
                output.setBackground(Color.yellow);
        
                add(inputPrompt);    // put prompt on applet
                add(input);          // put input on applet
                add(output);         // put output on applet
        } // init()
    
        // function for finding next prime number
        public void findPrime() {   
                // get input number
                n = Integer.parseInt(input.getText());
                z = n;
                do {
                        z++;
                        k = 2;
                        divisorFound = false;  
                                // so far no divisor of z has been found
                        while ((!divisorFound) && (k*k <= z)) {
                                // check if k is a divisor of z
                                if (z % k == 0) {
                                        divisorFound = true;
                                }
                                k++;
                        } //end while
                } while (divisorFound);
    
                // define the output string
                String str = "Die n\u00e4chstgr\u00f6\u00dfere Primzahl nach "  
                        + n + " ist " + z + '.';

                // put the sting into the output field      
                output.setText( str );  
        } // findPrime()
} // Primzahl