大四实习准备1_java构造器_android ListView

2015-4-23

Java构造器

与类名同名;无返回值(void也不行);被不同的修饰符修饰是有区别的;当构造函数被private修饰时，只有本类可访问。其他类可以通过该类的get函数得到对象。如单子模式;子类的构造函数默认调用super()，即父类的构造函数，然后再回到子类自己的构造函数。子类的构造函数中写了super()效果也一样，相当于没写(只能写在第一句，否则出错);构造函数可以重载(当父类有多个构造函数时，可以用super(xxx)来指定调用父类的哪个构造函数);当自定义了构造函数后，系统就不自动生成构造函数了。所以最好为每个类写无参构造函数，以免出错;运行过程：有继承关系时，1>初始化父类的静态块和静态成员变量 2>初始化子类的静态块和静态成员变量 3>执行父类的构造函数 4>执行子类的构造函数;

单子模式，是指一段时间内该类的对象只存在一份。

如

 //test.java
 class ace{
     private static ace a;
     private ace(){
         System.out.println("1");
     }
     public static ace getAce(){
         if( null == a ){
             a = new ace();
         }
         return a;
     }
 }
 
 public class test{
     public static void main(String[] args) {
         //ace a = new ace();错
         ace b = ace.getAce();
         ace c = ace.getAce();
 
         if( b==c )
             System.out.println("equal");
     }
 }

单子模式

android ListView

1.简单的使用库本身的布局

把数据和布局塞到适配器里，把适配器塞到ListView里。

     String[] data = {"1","2","3","4","5","6","7","8","9","1","2","3","4","5","6","7","8","9"};
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this,
                 android.R.layout.simple_list_item_1,data);
         ListView lv = (ListView)findViewById(R.id.listview);
         lv.setAdapter(adapter);
     };

 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context="${relativePackage}.${activityClass}" >
 
     <ListView
         android:id="@+id/listview"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         ></ListView>
 </RelativeLayout>

2.自定义每项的布局

自定义了布局diy_list_item、每一行数据的类fruit、继承自ArrayAdapter<fruit>的类fruitAdapter

 <?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="horizontal" >
     <ImageView
         android:id="@+id/imageView"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         />
     <TextView
         android:id="@+id/textView"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         />
 
 </LinearLayout>

diy_list_item.xml

 package com.example.listviewtest;
 
 public class fruit {
     private String name;
     private int imageId;
 
     public fruit(String name,int imageId){
         this.name = name;
         this.imageId = imageId;
     }
     public String getName(){
         return name;
     }
     public int getImageId(){
         return imageId;
     }
 }

fruit.class

 package com.example.listviewtest;
 
 import java.util.List;
 
 import android.content.Context;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.ArrayAdapter;
 import android.widget.ImageView;
 import android.widget.TextView;
 
 public class fruitAdapter extends ArrayAdapter<fruit>{
 
     private int resourceId;
     public fruitAdapter(Context context, int resource, List<fruit> objects) {
         super(context, resource, objects);
         // TODO Auto-generated constructor stub
         resourceId = resource;
     }
 
      @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         // TODO Auto-generated method stub
         fruit f = getItem(position);
         View v ;
         v = LayoutInflater.from(getContext()).inflate(resourceId, null);
         ImageView fruitImage = (ImageView)v.findViewById(R.id.imageView);
         TextView fruitName = (TextView)v.findViewById(R.id.textView);
         fruitImage.setImageResource(f.getImageId());
         fruitName.setText(f.getName());
         return v;
     }
 }

fruitAdapter.class

 package com.example.listviewtest;
 
 import java.util.ArrayList;
 
 import android.app.Activity;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.widget.AdapterView;
 import android.widget.AdapterView.OnItemClickListener;
 import android.widget.ArrayAdapter;
 import android.widget.ListView;
 import android.widget.Toast;
 
 public class MainActivity extends Activity {
 
     ArrayList<fruit> fruitList = new ArrayList<fruit>();
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         initFruit();
         fruitAdapter adapter = new fruitAdapter(MainActivity.this,
                 R.layout.diy_list_item,fruitList);
         ListView lv = (ListView) findViewById(R.id.listview);
         lv.setAdapter(adapter);
     }
 
     private void initFruit(){
         fruit one = new fruit("1",R.drawable._1);
         fruitList.add(one);
         fruit two = new fruit("2",R.drawable._2);
         fruitList.add(two);
         fruit three = new fruit("3",R.drawable._3);
         fruitList.add(three);
         fruit four = new fruit("4",R.drawable._4);
         fruitList.add(four);
         fruit five = new fruit("5",R.drawable._5);
         fruitList.add(five);
         fruit six = new fruit("6",R.drawable._6);
         fruitList.add(six);
         fruit seven = new fruit("7",R.drawable._7);
         fruitList.add(seven);
         fruit eight = new fruit("8",R.drawable._8);
         fruitList.add(eight);
         fruit nine = new fruit("9",R.drawable._9);
         fruitList.add(nine);
     }
 }

MainActivity.class

问题：

1>diy_list_item.xml里线性布局的宽和高都是match_parent，会不会太大了，留出空白？

2>为什么要写一个fruitAdapter类?(当然这里重写了getView()方法，是可以实现目标的。但不写这个类，用原有的适配器，可以吗？)

Hint: "_1"、"_2"等图片要自己下载，放在res/drawalbe-hdpi下。

3.在public View getView(int position, View convertView, ViewGroup parent)中利用convertView这个缓存来进行优化

 public View getView(int position, View convertView/*用于将之前加载好的布局进行缓存，以便重用*/, ViewGroup parent) {
         // TODO Auto-generated method stub
         fruit f = getItem(position);//该position对应的fruit对象
         View v ;//理解是diy_list_item这个布局，布局也是一种view
         ViewHolder vHolder = null;//用于缓存fruitImage和fruitName，避免每次都findViewById()
         if( null == convertView ){
             v = LayoutInflater.from(getContext()).inflate(resourceId, null);
             vHolder = new ViewHolder();
             vHolder.fruitImage = (ImageView)v.findViewById(R.id.imageView);
             vHolder.fruitName = (TextView)v.findViewById(R.id.textView);
             v.setTag(vHolder);
         }
         else{
             v = convertView;
             vHolder = (ViewHolder) v.getTag();
         }
         vHolder.fruitImage.setImageResource(f.getImageId());
         vHolder.fruitName.setText(f.getName());
         return v;
     }
      //内部类ViewHolder
      class ViewHolder{
          ImageView fruitImage;
          TextView fruitName;
      }

getView()

4.添加点击事件

 lv.setOnItemClickListener(new OnItemClickListener() {
             @Override
             public void onItemClick(AdapterView<?> parent, View view, int position,long id) {
                 // TODO Auto-generated method stub
                 fruit f = fruitList.get(position);
                 Toast.makeText(MainActivity.this, f.getName(), Toast.LENGTH_SHORT).show();
             }
         });