如何解决android中的NullPointer异常？

Question

This is a simple app in android. It shows nullpointerexception while executing. I can't get the value from the edittext . Here I just want to get the value from the edittext . But it shows nullpointerexception ..

how to solve this?

public class Calci extends Activity  {
    Button add;
    String str1;
    String str2;
    EditText txt1;
    EditText txt2;
    TextView tv;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calci);

        add=(Button) findViewById(R.id.button1);
        txt1=(EditText) findViewById(R.id.editText1);
        str1=txt1.getText().toString();
        txt2=(EditText) findViewById(R.id.editText2);
        str2=txt2.getText().toString();
        tv=(TextView) findViewById(R.id.textView4);

        addition();

        add.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {
                addition(); 
            }
        });
    }


    private void addition(){
        int res=0;
        res=Integer.parseInt(str1)+Integer.parseInt(str2);
        tv.setText(String.valueOf(res));
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_calci, menu);
        return true;
    }
}

Answer 1

Just remove addition(); from onCreate() . As I have doubt when its first time called your EditTexts are empty.

Also when you call addition() method you have to get the text from your both EditTexts .

And your method addition() something like,

 private void addition()
 {

  int res=0;
  str1=txt1.getText().toString();
  str2=txt2.getText().toString();
  res=Integer.parseInt(str1)+Integer.parseInt(str2);
  tv.setText(String.valueOf(res));
 }

Answer 2

make sure you are setting default values for editText1 and editText2 in xml or if you want to access values enter by user then chnage your code as and remove addition() before setOnClickListener :

      //  addition();  comment this line 
        add.setOnClickListener(new View.OnClickListener() 
        {

            public void onClick(View v) 
            {
                  str1=txt1.getText().toString();
                  str2=txt2.getText().toString();
                  addition(); 

            }
        });

Answer 3

Just replace the additiion() method in your oncreate() and change the method as like below

private void addition()
  {
         str1= str1==null?"0":str1;// do like this, so that if str1 returns null then it will be replaced by 0
         str2= str2==null?"0":str2;
         int res=0;
         res=Integer.parseInt(str1)+Integer.parseInt(str2);
         tv.setText(String.valueOf(res));
  }

Answer 4

You do not call addition() this method here :

        str2=txt2.getText().toString();
        tv=(TextView) findViewById(R.id.textView4);

        //addition();


        add.setOnClickListener(new View.OnClickListener()