请在运行程序时帮助获取java.lang.nullpointerexception

帮我解决这个代码，我得到运行时异常错误java.lang.nullpointerexception plz帮助我解决这个问题，以及如何使用java程序访问桌面上的任何文件， 就像我想访问桌面或任何其他文件夹中的任何图像或文本文件一样，我应该编写什么代码。仅供参考努布

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

---

您正在比较

i

，但未对其进行初始化

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

String i=null
if(i.equals("jarvis you there?"))
  ^^^^
Here there is NPE

你们能做的最好的事情就是把常数放在等式的左边

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

if("jarvis you there?".equals(i))
{
   ...

变量

i

为

null

，但您尝试取消引用它。表演

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

i=br.readLine();

---

在比较其值之前。

代码按照您编写它的顺序执行。如果要将用户输入与

If

中的某个值进行比较，则必须先读取该值，然后再进行比较：

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

System.out.println("Command me: ");
i = br.readLine();
if(i.equals("jarvis you there?"))
{
    System.out.println("at your service sir");
}

由于

readLine（）

可以返回null，因此仍然可能会出现null指针异常。防止这种情况发生的最简单方法是切换

equals

的两侧。文字字符串是对象，因此它们具有类似于

equals

的方法，但它们不能为null

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

if("jarvis you there?".equals(i))

接下来您会发现，您的程序读取一行并检查它是否仅是第一个命令，读取下一行并检查它是否是第二个命令，依此类推。要获取

“run paint”

命令，必须将其作为第4行输入

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

您应该将同一行与每个可能的命令进行比较。例如，使用

if。。否则如果。。。否则如果。。。其他

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
for (;;) {
    System.out.println("Command me: ");
    String line = br.readLine();
    if ("foo".equals(line)) {
        // do foo
    } else if ("bar".equals(line)) {
        // do bar
    } else if ("baz".equals(line)) {
        // do baz
    } else {
        System.out.println("Unknown Command. Bye.");
        break;
    }
}

或者在Java 7中使用

开关

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}

    switch (line) {
        case "foo":
            // do foo
            break;
        case "bar":
            // do foo
            break;
        //...
        default: 
            // } else {
            break;
    }

或者以奇特的面向对象风格：

import java.io.*;
class jarvis
{
public static void main(String args[])
{
String i=null,j=null,k=null,l=null,m=null,n=null,o=null,a=null,q=null;
try
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Command me: ");
if(i.equals("jarvis you there?"))
{
i=br.readLine();

System.out.println("at your service sir");
}
System.out.println("Command me: ");
if(j.equals("run command prompt"))
{
j=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start");
}
System.out.println("Command me: ");
if(k.equals("run notepad"))
{
k=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("notepd.exe");
}
System.out.println("Command me: ");
if(l.equals("run paint"))
{
l=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("mspaint.exe");
}
System.out.println("Command me: ");
if(m.equals("open facebook"))
{
m=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.facebook.com");
}
System.out.println("Command me: ");
if(n.equals("open google"))
{
n=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.google.com");
}
System.out.println("Command me: ");
if(o.equals("open youtube"))
{
o=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.youtube.com");
}
System.out.println("Command me: ");
if(a.equals("open yahoo"))
{
a=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.yahoo.com");
}
System.out.println("Command me: ");
if(q.equals("open omegle"))
{
q=br.readLine();
Runtime r=Runtime.getRuntime();
Process p=r.exec("cmd.exe /c start www.omegle.com");
}
}
catch(Exception e)
{
System.out.print(e);
}
}
}