Question

我是Android编码的新手，在编程方面经验太少。我想尝试制作一个计算器。我觉得它很容易但是，我被卡住了。基本上你输入一个EditText数字，然后按＆＃34; +＃* /＆＃34;然后输入另一个号码，然后按Enter键。但不知怎的，只要我按+或者它就崩溃了。谁能告诉我有什么问题？哦，logcat说：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
at com.industriesoi.taschenrechner.Calc.calcPlus(Calc.java:87)

但我不知道该改变什么..

这是我的代码：

package com.industriesoi.taschenrechner;

import com.industriesoi.taschenrechner.util.SystemUiHider;

import android.annotation.TargetApi;
import android.app.Activity;
import android.content.DialogInterface;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;


/**
 * An example full-screen activity that shows and hides the system UI (i.e.
 * status bar and navigation/system bar) with user interaction.
 *
 * @see SystemUiHider
 */
public class Calc extends Activity {

    // INITIAL
    EditText etZahl;
    TextView tvErgebnis;

    double zahl1=0;
    double zahl2=0;
    char operation = '0';
    double ergebnis=0;


    /**
     * Whether or not the system UI should be auto-hidden after
     * {@link #AUTO_HIDE_DELAY_MILLIS} milliseconds.
     */
    private static final boolean AUTO_HIDE = true;

    /**
     * If {@link #AUTO_HIDE} is set, the number of milliseconds to wait after
     * user interaction before hiding the system UI.
     */
    private static final int AUTO_HIDE_DELAY_MILLIS = 3000;

    /**
     * If set, will toggle the system UI visibility upon interaction. Otherwise,
     * will show the system UI visibility upon interaction.
     */
    private static final boolean TOGGLE_ON_CLICK = true;

    /**
     * The flags to pass to {@link SystemUiHider#getInstance}.
     */
    private static final int HIDER_FLAGS = SystemUiHider.FLAG_HIDE_NAVIGATION;

    /**
     * The instance of the {@link SystemUiHider} for this activity.
     */
    private SystemUiHider mSystemUiHider;



// ONCREATE HERE
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        etZahl = (EditText)findViewById(R.id.etZahl);
        tvErgebnis = (TextView)findViewById(R.id.tvErgebnis);
        setContentView(R.layout.activity_calc);

    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);

        // Trigger the initial hide() shortly after the activity has been
        // created, to briefly hint to the user that UI controls
        // are available.

    }

    public void calcPlus(View view){
        String stringZahl1 = etZahl.getText().toString();
        zahl1 = Double.parseDouble(stringZahl1);
        operation = '+';
        etZahl.setText("");
    }

    public void calcMinus(View view){
        String stringZahl1 = etZahl.getText().toString();
        zahl1 = Double.parseDouble(stringZahl1);
        operation = '-';
        etZahl.setText("");
    }

    public void calcMultiply(View view){
        String stringZahl1 = etZahl.getText().toString();
        zahl1 = Double.parseDouble(stringZahl1);
        operation = '*';
        etZahl.setText("");
    }

    public void calcDivide(View view){
        String stringZahl1 = etZahl.getText().toString();
        zahl1 = Double.parseDouble(stringZahl1);
        operation = '/';
        etZahl.setText("");
    }

    public void calcEnter(View view){
        //first operate;
        switch(operation){
            case '+':
                ergebnis=zahl1+zahl2;
                break;
            case '-':
                ergebnis=zahl1-zahl2;
                break;
            case '*':
                ergebnis=zahl1*zahl2;
                break;
            case '/':
                ergebnis=zahl1/zahl2;
                break;
            default:
                ergebnis=0;
        }
        //then change text;
        String doubleToString = Double.toString(ergebnis);
        tvErgebnis.setText(doubleToString);
    }
}

_ 哦，如果你能给我一些关于编码的建议那也很好。就像我怎么能改进这个计算器，即使它没什么用呢？

计算器不工作。编码新手

0 个答案: