Question

java:Cannot refer to a non-final variable tx inside an inner class defined in a different method

public class ActionIf extends JFrame {
private static final long serialVersionUID = 1L;
User u=null;
JTextField tx=null;
JButton jb=null;
private void showtext(){

JTextField tx=new JTextField();
User u=new User();
JButton jb=new JButton();
this.setSize(300,300);
this.setLayout(null);
tx.setSize(50,50);
tx.setBounds(50,50,50,50);
add(tx);
jb.setBounds(100,100,50,50);
jb.addActionListener(new java.awt.event.ActionListener(){

public void actionPerformed(java.awt.event.ActionEvent e) {
String str=tx.getText();

}

});
tx不能写在那里吗？

Answer 1

final JTextField tx = null;
// 使用匿名内部类，隐式调用外部变量，外部变量需要final修饰。
jb.addActionListener(new java.awt.event.ActionListener(){
    public void actionPerformed(java.awt.event.ActionEvent e) {
	String str = tx.getText();
}

Answer 2

就是说匿名内部类里面用到外面的变量时候，需要把外面的变量定义成final类型，也就是常量类型。
JTextField tx=new JTextField();前面加上final就可以了