null+null为什么等于nullnull<已解决> - 黑马程序员技术交流社区

import java.awt.*;
import java.awt.event.*;
import java.io.*;
class test
{
private Frame f;
private File fe;
private MenuBar mb;
private Menu m;
private MenuItem mload,msave,mclose;
private TextArea ta;
test()
{
ini();
}
public void ini()
{
f=new Frame();
ta=new TextArea();
mb=new MenuBar();
m=new Menu("文件");
mload=new MenuItem("打开");
msave=new MenuItem("保存");
mclose=new MenuItem("关闭");
f.setBounds(200,100,500,400);
f.setLayout(new FlowLayout());
f.add(ta);
f.setMenuBar(mb);
mb.add(m);
m.add(mload);
m.add(msave);
m.add(mclose);
method();
f.setVisible(true);
}
public void method()
{
f.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}});
mclose.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
System.exit(0);
}
});
mload.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
try{
FileDialog fdg=new FileDialog(f,"",FileDialog.LOAD);
fdg.setVisible(true);
String dirpath=fdg.getDirectory();
String filepath=fdg.getFile();
if(dirpath==null||filepath==null){
//System.out.println(dirpath+"::::"+filepath);
return;}
else{
//System.out.println(dirpath+"::::"+filepath);
ta.setText("");
fe=new File(dirpath+filepath);
byte[] b=new byte[1024*1024];
int len;
BufferedInputStream bis=new BufferedInputStream(new FileInputStream(fe));
while((len=bis.read(b))!=-1)
{
ta.append(new String(b,0,len)+"\r\n");
}}
}
catch (Exception ex)
{
throw new RuntimeException("。。。。");
}
}
}
);
msave.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
try
{
String textstr=ta.getText();
if(fe==null)
{
FileDialog fdg=new FileDialog(f,"",FileDialog.SAVE);
fdg.setVisible(true);
String path=fdg.getDirectory()+fdg.getFile();
// System.out.println(fdg.getDirectory());打印结果为null；
// System.out.println(fdg.getFile());打印结果为null；
// System.out.println(path);打印结果为nullnull；
if(path==null))//这步判断无效....
return;
else
fe=new File(path);
}
PrintStream ps=new PrintStream(new FileOutputStream(fe),true);
ps.println(textstr);
}
catch (Exception ex)
{
throw new RuntimeException("；；；；");
}
}
});
}
public static void main(String[] args)
{
new test();
}
}

复制代码