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社区首页 >问答首页 >Android studio如何在空对象引用（HTTP请求）上调用虚方法'java?

问Android studio如何在空对象引用（HTTP请求）上调用虚方法'java?
EN

Stack Overflow用户
提问于 2018-08-09 04:54:47
回答 1查看 0关注 0票数 0

java：

代码语言：txt
复制
package com.mayus.crowdwash_android;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import java.net.URL;

import static com.mayus.crowdwash_android.WebContent.web;

public class Login extends AppCompatActivity {

    static Login login;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        login = this;
        setContentView(R.layout.activity_login);


        Button debug = findViewById(R.id.debug);
        debug.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent client = new Intent(Login.this, Client.class);
                startActivity(client);
            }
        });
        Button signin = Login.getInstance().findViewById(R.id.button3);
        signin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                    System.out.println("Klick!");
                    //...
                    try {
                        //CrowdwashUser user = WebContent.getUserFromJson(WebContent.getLoginUser("Mayus", "TallerikIstKaka"));
                        WebContent web = new WebContent();
                        String user = web.getUserFromJson("Mayus", "TallerikIstKaka");

                        System.out.println(user);

                    } catch (Exception e) {
                        e.printStackTrace();
                    }


                    if (false) {
                        Intent client = new Intent(Login.getInstance(), Client.class);
                        Login.getInstance().startActivity(client);
                    }


            }
        });



    }

    public static Login getInstance() {
        return login;
    }
}

java：

代码语言：txt
复制
package com.mayus.crowdwash_android;

import android.content.Intent;
import android.os.AsyncTask;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.URL;
import java.nio.charset.Charset;

public class WebContent extends AsyncTask<URL, Integer, JSONObject> {

    JSONObject obj;
    static WebContent web;
    static android.view.View.OnClickListener listener;

    private static String readAll(Reader rd) throws IOException {
        StringBuilder sb = new StringBuilder();
        int cp;
        while ((cp = rd.read()) != -1) {
            sb.append((char) cp);
        }
        return sb.toString();
    }

        public WebContent() {
            web = this;


        }


            //public String getUserFromJson(String name, String pw) throws Exception {
                /**WebContent webContent = new WebContent();
                 webContent.execute(new URL("http://tallerikyt.ddns.net:5000/login?user=" + "Mayus" + "&pw=" + "TallerikIstKaka"), null, null).get();
                 //new WebContent().execute(new URL("http://tallerikyt.ddns.net:5000/login?user=" + name + "&pw=" + pw), null, null);

                 if (obj.getBoolean("error")) {
                 return "";
                 } else {
                 return obj.getString("SessionID");
                 }**/
           // }
            /**
            public String getUserFromJson(String name, String pw) throws Exception {
            new WebContent().execute(new URL("http://tallerikyt.ddns.net:5000/login?user=" + name + "&pw=" + pw), null, null).get();
            if (obj.getBoolean("error")) {
                return "";
            } else {
                return obj.getString("SessionID");
            }**/

            public String getUserFromJson(String name, String pw) throws Exception {
                JSONObject data = new WebContent().execute(new URL("http://tallerikyt.ddns.net:5000/login?user=" + name + "&pw=" + pw), null, null).get();
                Log.d("From server" , data.toString());
                if (data.getBoolean("error")) {
                    return "";
                } else {
                    return data.getString("SessionID");
                }
            }




            @Override
            protected JSONObject doInBackground(URL... urls) {
                InputStream is = null;
                try {
                    is = urls[0].openStream();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    BufferedReader rd = new BufferedReader(new InputStreamReader(is, Charset.forName("UTF-8")));
                    String jsonText = readAll(rd);
                    System.out.println("Test" + jsonText);
                    JSONObject json = new JSONObject(jsonText);
                    obj = json;
                    return json;
                } catch (JSONException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        is.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                return null;
            }
            public static WebContent getWeb() { return web; }




}

Former error

代码语言：txt
复制
07-30 21:00:17.788 15452-15452/com.mayus.crowdwash_android W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.mayus.crowdwash_android.WebContent.getUserFromJson(java.lang.String, java.lang.String)' on a null object reference
        at com.mayus.crowdwash_android.Login$2.onClick(Login.java:37)
        at android.view.View.performClick(View.java:6291)
        at android.view.View$PerformClick.run(View.java:24931)
        at android.os.Handler.handleCallback(Handler.java:808)
        at android.os.Handler.dispatchMessage(Handler.java:101)
        at android.os.Looper.loop(Looper.java:166)
07-30 21:00:17.789 15452-15452/com.mayus.crowdwash_android W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:7425)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)

Older Classes:

代码语言：txt
复制
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class Login extends AppCompatActivity {

static Login login;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    login = this;
    setContentView(R.layout.activity_login);


    Button debug = findViewById(R.id.debug);
    debug.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent client = new Intent(Login.this, Client.class);
            startActivity(client);
        }
    });
    Button signin = Login.getInstance().findViewById(R.id.button3);
    signin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

                System.out.println("Klick!");

                try {
                    //CrowdwashUser user = WebContent.getUserFromJson(WebContent.getLoginUser("Mayus", "TallerikIstKaka"));
                    String user = WebContent.getWeb().getUserFromJson("Mayus", "TallerikIstKaka");
                    System.out.println(user);

                } catch (Exception e) {
                    e.printStackTrace();
                }


                if (false) {
                    Intent client = new Intent(Login.getInstance(), Client.class);
                    Login.getInstance().startActivity(client);
                }


        }
    });



}

public static Login getInstance() {
    return login;
}

}`WebContent.java：

代码语言：txt
复制
    package com.mayus.crowdwash_android;

import android.content.Intent;
import android.os.AsyncTask;
import android.view.View;
import android.widget.Button;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.URL;
import java.nio.charset.Charset;

public class WebContent extends AsyncTask<URL, Integer, JSONObject> {

    JSONObject obj;
    static WebContent web;
    static android.view.View.OnClickListener listener;

    private static String readAll(Reader rd) throws IOException {
        StringBuilder sb = new StringBuilder();
        int cp;
        while ((cp = rd.read()) != -1) {
            sb.append((char) cp);
        }
        return sb.toString();
    }

        public WebContent() {
            web = this;


        }


            public String getUserFromJson(String name, String pw) throws Exception {
                new WebContent().execute(new URL("http://tallerikyt.ddns.net:5000/login?user=" + name + "&pw=" + pw), null, null);


                if (obj.getBoolean("error")) {
                    return "";
                } else {
                    return obj.getString("SessionID");
                }

            }

            @Override
            protected JSONObject doInBackground(URL... urls) {
                InputStream is = null;
                try {
                    is = urls[0].openStream();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                try {
                    BufferedReader rd = new BufferedReader(new InputStreamReader(is, Charset.forName("UTF-8")));
                    String jsonText = readAll(rd);
                    JSONObject json = new JSONObject(jsonText);
                    obj = json;
                    return json;
                } catch (JSONException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        is.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                return null;
            }
            public static WebContent getWeb() { return web; }




}

java：

代码语言：txt
复制
package com.mayus.crowdwash_android;

import java.io.IOException;

public class CrowdwashUser {
    String Name = "";
    String SessionID = "";
    boolean login = false;

    public CrowdwashUser(String name, String session) {
        System.out.println("CrowdwashUser");
        this.Name = name;
        this.SessionID = session;
        login =  true;
    }
    public CrowdwashUser() {
        login = false;
    }

    public String getUserName() {
            return Name;

    }

    public String getSessionID() {
        return SessionID;
    }

    public boolean isLogin() {

        return login;
    }
}
EN

回答 1

Stack Overflow用户

发布于 2018-08-09 14:04:11

调用String user = WebContent.getWeb().getUserFromJson("Mayus", "TallerikIstKaka");方法，WebContent().getWeb()返回静态数据成员，该成员将在WebContent类被创建。

代码语言：txt
复制
WebContent web=new WebContent();
String user=web.getUserFromJson("Mayus", "TallerikIstKaka");`
票数 0
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/-100002041

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