在JFrame中显示.png图像？

Question

我有点卡住了。为什么不这样做？我刚收到一个错误说：

  

java.lang.NoSuchMethodError：main

     

线程“main”中的异常

import java.awt.*; 
import javax.swing.*; 

@SuppressWarnings("serial")
public class ShowPNG extends JFrame
{    

  public void main(String arg) 
  { 
    if (arg == null ) {
        arg = "C:/Eclipse/workspace/ShowPNG/bin/a.png";
    }      
    JPanel panel = new JPanel(); 
    panel.setSize(500,640);
    panel.setBackground(Color.CYAN); 
    ImageIcon icon = new ImageIcon(arg); 
    JLabel label = new JLabel(); 
    label.setIcon(icon); 
    panel.add(label);
    this.getContentPane().add(panel); 
    this.setVisible(true);
  }

}

Answer 1

您的主要方法应该是：

public static void main(String[] args)

Answer 2

main需要是静态的，并且必须有String []的参数，而不是String。

要在构造函数中修复此内容，例如

import java.awt.*; 
import javax.swing.*; 

@SuppressWarnings("serial")
public class ShowPNG extends JFrame
{    
  private ShowPNG(String arg){
      if (arg == null ) {
        arg = "C:/Eclipse/workspace/ShowPNG/bin/a.png";
    }      
    JPanel panel = new JPanel(); 
    panel.setSize(500,640);
    panel.setBackground(Color.CYAN); 
    ImageIcon icon = new ImageIcon(arg); 
    JLabel label = new JLabel(); 
    label.setIcon(icon); 
    panel.add(label);
    this.getContentPane().add(panel); 
  }
  public static void main(String[] args) {
      new ShowPNG(args.length == 0 ? null : args[0]).setVisible(true); 
  }
}

Answer 3

这是完成的代码：

import java.awt.*;  
import javax.swing.*;  

@SuppressWarnings("serial") 
public class ShowPNG extends JFrame {   

  public ShowPNG(String argx) { 
    if ( argx == null ) {
      argx = "a.png";
 }   
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
 this.setSize(500,640);
    JPanel panel = new JPanel();  
    //panel.setSize(500,640);
    panel.setBackground(Color.CYAN);  
    ImageIcon icon = new ImageIcon(argx);  
    JLabel label = new JLabel();  
    label.setIcon(icon);  
    panel.add(label); 
    this.getContentPane().add(panel);    
  } 

  public static void main(String[] args) { 
      new ShowPNG(args.length == 0 ? null : args[0]).setVisible(true);
  } 

}