<?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">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

        <Button
            android:id="@+id/btn_load"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="加载网络图片" />

        <Button
            android:id="@+id/btn_size_load"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="加载网络图片(设置大小)" />

        <Button
            android:id="@+id/btn_err_load"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="加载网络图片（设置出错图片）" />
    </LinearLayout>


    <ImageView
        android:id="@+id/iv_img"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>

package com.example.picassodemo;

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

import com.squareup.picasso.Picasso;

public class MainActivity extends AppCompatActivity {
    private Button btnLoad, btnSizeLoad, btnErrLoad;
    private ImageView ivImg;
    private String url = "http://i.zeze.com/attachment/forum/201605/06/214815xnd5dz5t58fndt85.jpg";

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        initEvent();


    }

    private void initView() {
        btnLoad = (Button) findViewById(R.id.btn_load);
        btnSizeLoad = (Button) findViewById(R.id.btn_size_load);
        btnErrLoad = (Button) findViewById(R.id.btn_err_load);
        ivImg = (ImageView) findViewById(R.id.iv_img);
    }

    private void initEvent() {
        btnLoad.setOnClickListener(myOnClickListener);
        btnSizeLoad.setOnClickListener(myOnClickListener);
        btnErrLoad.setOnClickListener(myOnClickListener);
    }

    private View.OnClickListener myOnClickListener = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            switch (v.getId()) {
                case R.id.btn_load:
                    //加载网络图片
                    Picasso.with(MainActivity.this).load(url).into(ivImg);
                    break;
                case R.id.btn_size_load:
                    //加载网络图片（设置大小）
                    Picasso.with(MainActivity.this).load(url).resize(120, 120).into(ivImg);
                    break;
                case R.id.btn_err_load:
                    //加载网络图片（设置错误图片）
                    Picasso.with(MainActivity.this).load(url).error(R.mipmap.ic_launcher).into(ivImg);
                    break;
            }
        }
    };
}