Webview后退按钮调用 - null对象引用

Question

这是一个使用简单的android webview函数的测试应用程序。但是当我按下按钮返回上一个网页时，我在下面遇到了fatal error;

“java.lang.NullPointerException：尝试在空对象引用上调用虚方法'boolean android.webkit.WebView.canGoBack（）'”

主页(MainActivity.java)和Web查看页面(WebviewPage.java)。我试图通过谷歌搜索来修复这个错误...但我不能。

提前致谢。

main activity.Java

import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.KeyEvent;
import android.view.View;

public class MainActivity extends AppCompatActivity {

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

        Intent intent = new Intent(this, LoadingActivity.class);
        startActivity(intent);}

    public void onClick(View view)     {
        Intent intent1 = new Intent (this, WebViewPage.class);
        intent1.putExtra("name","https://Google.com"); 
        startActivity(intent1);
    } 

}

Web view Page.Java

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class WebViewPage extends Activity {
    Intent intent;
    WebView webView;

    //key down
    @Override
    public void onBackPressed() {
        if (webView.canGoBack()) {
            webView.goBack();
        } else {
            super.onBackPressed();
        }
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.web_view);

        WebView webView =(WebView) findViewById(R.id.webView);
        webView.setWebViewClient(new WebViewClient());

        WebSettings webSettings = webView.getSettings();
        webSettings.setJavaScriptEnabled(true);

        intent = getIntent();
        String name = intent.getExtras().getString("name");

        webView.loadUrl(name);

        Toolbar mToolbar = (Toolbar) findViewById(R.id.toolbar);
        mToolbar.setTitle(getString(R.string.app_name));
        mToolbar.setNavigationIcon(R.drawable.ic_home_black_24dp);

        mToolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }
}

Answer 1

你在课堂内和'onCreate'里面有两个webView变量

删除onCreate中的webView变量。

例）

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class WebViewPage extends Activity {
    Intent intent;
    WebView webView;

    //key down
    @Override
    public void onBackPressed() {
        if (webView.canGoBack()) {
            webView.goBack();
        } else {
            super.onBackPressed();
        }
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.web_view);

        webView = (WebView) findViewById(R.id.webView);
        webView.setWebViewClient(new WebViewClient());

        WebSettings webSettings = webView.getSettings();
        webSettings.setJavaScriptEnabled(true);

        intent = getIntent();
        String name = intent.getExtras().getString("name");

        webView.loadUrl(name);

        Toolbar mToolbar = (Toolbar) findViewById(R.id.toolbar);
        mToolbar.setTitle(getString(R.string.app_name));
        mToolbar.setNavigationIcon(R.drawable.ic_home_black_24dp);

        mToolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }
}