如何修复java.lang.NullPointerException（android）？

Question

Whenever I run my code I get an error that looks like it originates from line 14 of my code because of a java.lang.NullPointerException. Here's my code:

package com.example.android.rng;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

import java.util.Random;


public class MainActivity extends ActionBarActivity {
    // get EditText by id
    final EditText et=(EditText)findViewById(R.id.minimum);
    final EditText tv=(EditText)findViewById(R.id.maximum);
    // set Et as an integer
    int min = Integer.parseInt(et.getText().toString());
    int max = Integer.parseInt(tv.getText().toString());
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    /*
    * Generates a random number when the "Generate" button is pressed
    */
    public void randomNumGen (View View) {
        Random r = new Random();
        int i1 = r.nextInt(max - min + 1) + min;
        displayResults(i1);
    }
     /*
      * Displays RandomNumGen's product to the screen
      */
    public void displayResults (int results) {

        setContentView(R.layout.activity_main);
        TextView textView = (TextView) findViewById(R.id.results);
        textView.setText(results);
    }
}

I have scanned through my code multiple times but cannot seem to find the problem, and I'm pretty new to coding so I don't have much experience with most of the code I am using.

Answer 1

您的EditTexts为null，因为在您调用findViewById时，您的布局没有膨胀，因此它返回null，请在setContentView()之后在onCreate方法中调用它

Answer 2

You can't access objects in your layout until after the setContentView call.

Change the onCreate to this:

// get EditText by id
final EditText et;
final EditText tv;

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

    et=(EditText)findViewById(R.id.minimum);
    tv=(EditText)findViewById(R.id.maximum);

    // set Et as an integer
    int min = Integer.parseInt(et.getText().toString());
    int max = Integer.parseInt(tv.getText().toString());

Also, the way the code is now assumes that the two EditText fields will always contain numbers. If they start out empty and the user inputs numbers you should probably move the code where you get the data inside a button press or something similar.

Answer 3

final EditText et=(EditText)findViewById(R.id.minimum);
final EditText tv=(EditText)findViewById(R.id.maximum);
// set Et as an integer
int min = Integer.parseInt(et.getText().toString());
int max = Integer.parseInt(tv.getText().toString());

place this in onCreate function !