这是一个AWT程序，用于执行各种字符串操作，例如反向字符串，字符串大写/小写，长度

Question

这是一个Java AWT程序，用于执行各种字符串操作，例如反向字符串，字符串大写/小写，字符串长度。 我有错误

方法声明无效； 需要返回类型

import java.io.*;
import java.awt.*;
import java.awt.event.*;
class StringOperationsGUI extends Frame
{
    TextField tf1, tf2;
    public StringOperationsGUI()
    {
        setLayout(new FlowLayout());

        Label lb1=new Label("Enter String");
        Label lb2=new Label("Result is: ");

        tf1=new TextField(25);
        tf2=new TextField(25);

        Button btn1=new Button("Reverse");
        Button btn2=new Button("To Uppercase");
        Button btn3=new Button("To Lowecase");
        Button btn4=new Button("Length");

        add(lb1); add(tf1);
        add(lb2); add(tf2);
        add(btn1); add(btn2); add(btn3); add(btn4);

        btn1.addActionListener(new Inner1());
        btn2.addActionListener(new Inner2());
        btn3.addActionListener(new Inner3());
        btn4.addActionListener(new Inner4());
    }

    class Inner1 implements ActionListener
    {
        public static actionPerformed(ActionEvent ae)
        {
            StringBuffer sb=new StringBuffer(tf1.getText());
            tf2.setText(sb.reverse().toString());
        }
    }

    class Inner2 implements ActionListener
    {
        public static actionPerformed(ActionEvent ae)
        {
            String s=tf1.getText();
            tf2.setText(s.toUpperCase());
        }
    }

    class Inner3 implements ActionListener
    {
        public static actionPerformed(ActionEvent ae)
        {
            String s=tf1.getText();
            tf2.setText(s.toLowerCase());
        }
    }

    class Inner4 implements ActionListener
    {
        public static actionPerformed(ActionEvent ae)
        {
            String s=tf1.getText();
            tf2.setText(""+s.length());
        }
    }

    public static void main(String ar[])
    {
        StringOperationsGUI fr=new StringOperationsGUI();
        fr.setSize(300,300);
        fr.setTitle("Performing String Operations");
        fr.setVisible(true);
    }
}

我的输出显示了这一点