Question

当我点击添加按钮时，我收到空指针异常。它在AlertDialog.Builder a1 = new AlertDialog.Builder(c)行

上的第39行

package com.example.calculator;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class Calci extends Activity {
    TextView t1;
    EditText e1, e2;
    Button add, sub, mul, div;
    Context c;

    String b, a;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calci);
        e1 = (EditText) findViewById(R.id.EditText01);
        e2 = (EditText) findViewById(R.id.EditText02);
        add = (Button) findViewById(R.id.add);
        sub = (Button) findViewById(R.id.sub);
        mul = (Button) findViewById(R.id.mul);
        div = (Button) findViewById(R.id.div);
        t1 = (TextView) findViewById(R.id.textView1);
        a = e1.getText().toString();
        b = e2.getText().toString();
add.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View arg0) {
        if (a.matches("") || b.matches("")){
                AlertDialog.Builder a1 = new AlertDialog.Builder(c);


                // Setting Dialog Title
                a1.setTitle("Alert Dialog");

                // Setting Dialog Message
                a1.setMessage("PLEASE ENTER SOMETHING");

                a1.setPositiveButton("yes",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog,
                                    int button1) {
                                // if this button is clicked, close
                                // current activity
                                dialog.cancel();
                            }

                        });

                // Showing Alert Message
                AlertDialog alertDialog = a1.create();
                a1.show();

            }


            int result = Integer.parseInt(a) + Integer.parseInt(b);
            t1.setText(Integer.toString(result));


    }

});
    }
}

Answer 1

您没有初始化Context对象c。现在，初始化c如下...

Context c = this;

或者，如下所示......

Context c = Calci.this;

如果您不想使用Context对象c，请按以下方式初始化AlertDialog.Builder ...

AlertDialog.Builder a1 = new AlertDialog.Builder(Calci.this);

Answer 2

使用活动名称。而不是“c”。

Answer 3

将第39行更改为：

AlertDialog.Builder a1 = new AlertDialog.Builder(getApplicationContext());

“警报”对话框中出现“空指针异常”

3 个答案: