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");
    }
}

 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>

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;
    }
}

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;
    }
}

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);
    }
}

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;
     }

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();
            }            
        });