Question

我是编码和热心学习Android Studio的新手，接下来是视频编码。我已经阅读了那篇文章 - 什么是NullPointerException并且在更多相关文章中理解了这个概念，但是我无法解决虽然我理解错误是关于哪个代码，但如果没有，那么我的问题是什么。按下计算器应用程序上的任何按钮后，我面临这个NullPointerException：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                                           at com.blogeomics.calcee.MainActivity.numberPressed(MainActivity.java:167)
                                                                           at com.blogeomics.calcee.MainActivity$7.onClick(MainActivity.java:97)

代码如下：

package com.blogeomics.calcee;

import android.app.Activity;
import android.media.Image;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.TextView;

import static android.R.string.no;
import static android.os.Build.VERSION_CODES.O;

public class MainActivity extends AppCompatActivity {

    String runningNumber = "0";
    TextView resultsView;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button oneBtn = (Button) findViewById(R.id.button1);
        Button twoBtn = (Button) findViewById(R.id.button2);
        Button threeBtn = (Button) findViewById(R.id.button3);
        Button fourBtn = (Button) findViewById(R.id.button4);
        Button fiveBtn = (Button) findViewById(R.id.button5);
        Button sixBtn = (Button) findViewById(R.id.button6);
        Button sevenBtn = (Button) findViewById(R.id.button7);
        Button eightBtn = (Button) findViewById(R.id.button8);
        final Button nineBtn = (Button) findViewById(R.id.button9);
        Button zeroBtn = (Button) findViewById(R.id.button0);

        ImageButton calcBtn = (ImageButton) findViewById(R.id.buttonEqual);
        ImageButton divideBtn = (ImageButton) findViewById(R.id.buttonDivide);
        ImageButton multiplyBtn = (ImageButton) findViewById(R.id.buttonMultiply);
        ImageButton addBtn = (ImageButton) findViewById(R.id.buttonAdd);
        ImageButton subtractBtn = (ImageButton) findViewById(R.id.buttonSubtract);

        Button clearBtn = (Button) findViewById(R.id.buttonClear);
        TextView resultsView = (TextView) findViewById(R.id.result_view);

        resultsView.setText("");

        oneBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(1);

            }
        });

        twoBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(2);

            }
        });

        threeBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(3);
            }
        });

        fourBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(4);
            }
        });

        fiveBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(5);
            }
        });

        sixBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(6);
            }
        });

        sevenBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(7);
            }
        });

        eightBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(8);
            }
        });

        nineBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(9);
            }
        });

        zeroBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberPressed(0);
            }
        });

        calcBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        addBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        subtractBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        multiplyBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        divideBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

        clearBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });
    }

    void numberPressed(int number) {
        runningNumber = runningNumber + String.valueOf(number);
        resultsView.setText(runningNumber);


    }


}

我已经阅读了很多有关此错误的帖子，但仍然无法弄清楚如何解决我的问题，我知道这应该是一个小问题，但我无法理解，请帮忙。

Answer 1

更改此行：

TextView resultsView = (TextView) findViewById(R.id.result_view);

到

resultsView = (TextView) findViewById(R.id.result_view);

现在您的resultsView将是全局的

如何修复此NullPointError？

1 个答案: