请求网络要在子线程中进行，对UI的操作要放在主线程 

package com.example.picturefromnet;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends Activity implements View.OnClickListener {
    private ImageView iv_icon;
    private EditText et_url;
    private Button btn_submit;
    private final int SUCCESS = 0;
    private final int FAIL = 1;
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case SUCCESS:
                    Bitmap bitmap = (Bitmap) msg.obj;
                    iv_icon.setImageBitmap(bitmap);
                    //设置imageView显示的图片 break; case FAIL: 
                    Toast.makeText(MainActivity.this, "图片加载失败", Toast.LENGTH_LONG).show();
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv_icon = (ImageView) findViewById(R.id.iv_icon);
        et_url = (EditText) findViewById(R.id.et_url);
        btn_submit = (Button) findViewById(R.id.btn_submit);
        btn_submit.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        final String url = et_url.getText().toString().trim();
        new Thread(new Runnable() {
            @Override
            public void run() {
                Bitmap bitmap = getImageFromNet(url);
                if (bitmap != null) {
                    Message msg = new Message();
                    msg.what = SUCCESS;
                    msg.obj = bitmap;
                    handler.sendMessage(msg);
                } else {
                    Message msg = new Message();
                    msg.what = FAIL;
                    handler.sendMessage(msg);
                }
            }
        }).start();
    }

    private Bitmap getImageFromNet(String url) {
        HttpURLConnection conn = null;
        try {
            URL mURL = new URL(url);
            conn = (HttpURLConnection) mURL.openConnection();
            conn.setRequestMethod("GET"); //设置请求方法 
            // conn.setConnectTimeout(10000);设置连接服务器超时时间 
            conn.setReadTimeout(5000); //设置读取数据超时时间 
            conn.connect(); //开始连接
            int responseCode = conn.getResponseCode();
            //得到服务器的响应码 
            if (responseCode == 200) { //访问成功 
                InputStream is = conn.getInputStream(); //获得服务器返回的流数据 
                Bitmap bitmap = BitmapFactory.decodeStream(is); //根据流数据 创建一个bitmap对象 
                return bitmap;
            } else { //访问失败
                Log.d("lyf--", "访问失败===responseCode：" + responseCode);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (conn != null) {
                conn.disconnect(); //断开连接 
            }
        }
        return null;
    }
}

布局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#fff">

    <ImageView
        android:id="@+id/iv_icon"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"/>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:orientation="horizontal">

        <EditText
            android:id="@+id/et_url"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:text="http://mingxing.facang.com/uploads/allimg/150528/1022435210-1.jpg"
            android:singleLine="true"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/btn_submit"
            android:layout_width="50dp"
            android:layout_height="match_parent"
            android:text="GO"
            android:gravity="center"
            android:textSize="20sp"/>

    </LinearLayout>
</LinearLayout>

最后别忘了添加权限

<uses-permission android:name="android.permission.INTERNET"> </uses-permission>