一、起因

    采用下面的代码为对象数组里面的每个元素的字段赋值时报错。

public class StudentArray {
    public static void main(String[] args){
        Student[] stuArr = new Student[5]; //创建对象数组，stuArr存储地址

        for(int i = 0; i < stuArr.length; ++i){
            System.out.println(stuArr[i]); //null

            stuArr[i].number = i + 1; //报错
            stuArr[i].state = (int)Math.round(Math.random() * 3);
            stuArr[i].score = (int)Math.round(Math.random() * 100);
            System.out.println(stuArr[i]);
        }
    }
}

class Student{
    int number;
    int state;
    int score;

    @Override
    public String toString() {
        return "Student{" +
                "number=" + number +
                ", state=" + state +
                ", score=" + score +
                '}';
    }
}

    错误信息如下：自定义对象数组的元素的值为null，报空指针异常。

null
Exception in thread "main" java.lang.NullPointerException: Cannot assign field "number" because "stuArr[i]" is null
	at com.zhangqu.java.method.StudentArray.main(StudentArray.java:19)

    原因分析：因为Student[] stuArr = new Student[5];创建对象数组时，返回系统为该对象数组在堆区分配的空间的地址，并将地址值存储到引用变量stuArr，stuArr的数据类型为类数组Student[]，是引用类型；而该数组的每一个元素都是类Student的对象(本质是引用，存储null或地址值)，数组的每一个元素的数据类型为类Student，也是引用类型，由于没有继续使用new为这些引用类型的Student类对象分配空间，所有数组的元素默认被初始化为null。

    不较真的情况下，可以把Student[] stuArr = new Student[5];看做如下形式：

Student stu1 = null;
Student stu2 = null;
......
Student stu5 = null;
Student[] stuArr = {stu1, stu2, ..., stu5}

二、正确代码

public class StudentArray {
    public static void main(String[] args){
        Student[] stuArr = new Student[5]; //创建对象数组，stuArr存储地址, 初始化变量stuArr
        int num = 0;
        for (Student stu : stuArr) {
            System.out.println(stu); //null

            stu = new Student(); //用new分配空间，显式的为数组的每一个元素赋值,stu存储地址
            System.out.println(stu);
            System.out.println(stu.getClass().toString()); //获取数据类型

            stu.number = ++num;
            stu.state = (int)Math.round(Math.random() * 3);
            stu.score = (int)Math.round(Math.random() * 100);
            System.out.println(stu);
        }
    }
}

class Student{
    int number;
    int state;
    int score;

    @Override
    public String toString() {
        return "Student{" +
                "number=" + number +
                ", state=" + state +
                ", score=" + score +
                '}';
    }
}

运行结果：

null
Student{number=0, state=0, score=0}
class com.zhangqu.java.method.Student
Student{number=1, state=1, score=10}
null
Student{number=0, state=0, score=0}
class com.zhangqu.java.method.Student
Student{number=2, state=0, score=42}
......

    内存分析概要图如下：

    由于是自定义的类，所以创建对象数组时，第一步是用new为数组对象本身分配空间(初始化)，第二步是用new为数组对象的元素分配空间(赋值)。

public class StudentArray {
    public static void main(String[] args){
    }

    @Test
    void testObjArr(){
        StuObjArr[] stuObjArrs = new StuObjArr[2]; //第一步，为对象数组分配空间,初始化stuObjArrs
        for(int i = 0; i < stuObjArrs.length; i++){
            stuObjArrs[i] = new StuObjArr(); //第二步，为数组元素分配空间，为stuObjArrs[i]赋值
            System.out.println(stuObjArrs[i]); //打印数组元素在堆区的地址
        }
    }
}

class StuObjArr{
    int id;
    String name;
}

运行结果：

com.zhangqu.java.method.StuObjArr@2a70a3d8
com.zhangqu.java.method.StuObjArr@289d1c02

最后附上一张数据类型图，基础真的很重要。
引用类型的变量，只能存储两类值，null或地址值(含变量的数据类型）

const data={};定义一个数组；

const data={

person:[

                               { 'ysjkMc':'联调1',

'ysjkDm':'01',},

{

  'ysjkMc':'联调1',

'ysjkDm':'01',

}

]，

}

作为一个小白，最近有兴趣试了试小程序开发，发现小程序里面写js和写网页的JavaScript还是有些不同的。

emmm，赋值真的有些坑。

首先是赋值后，如果要渲染到前端，一定要用that.setData({ }),否则改了也是木有用滴。

然后就是对数组的赋值，直接在setData里面是不可以给数组元素动态赋值的，那么怎么解决嘞。来个代码看看。

js  关注点在n[],p[](命名不规范~~~ 懒得改了）

// pages/friend/friend.js

var app = getApp()

Page({



  /**

   * 页面的初始数据

   */

  data: {

    'inform':'一对一学车”，享受VIP教学！',

    flag: 'true',

    imgUrls: [],

    circleid:5,

    content:'',

    openid:'',

    stus:[],

    all:[],

    stu:[],

    comment:[],

    textdata:'',

    p:[],

    n:[],

  },



  /**

   * 生命周期函数--监听页面加载

   */

  onLoad: function (options) {

    var that = this;

    wx.getStorage({

      key: 'userInfo',

      success: function (res) {

        console.log("cheng");

        that.setData({

          userInfo: res.data

        })

      },

    });

    wx.getStorage({

      key: 'openid',

      success: function (res) {

        console.log(res)

        console.log(res.data)

        that.setData({

          openid: res.data

        })

      },

    });

    var array = new Array()


    for (let i = 1; i < 4; i++) {

      let url = "" + i + ".png"

      wx.downloadFile({

        url: url,

        success: (res) => {

          let temp = res.tempFilePath

          array[i - 1] = temp

          that.setData({

            imgUrls: array

          })

        }

      })



    }

 /*   wx.request({

      url: '',

      method: 'POST',

      header: { "Content-Type": 'application/json' },


      success: function (res) {


        that.setData({

          all: res.data.circleall,


        })

        console.log(that.data.all.length)



        for (var i = 0; i < that.data.all.length; i++) {

          that.data.stu[i] = that.data.all[i];

          console.log(that.data.stu[i].circleid)

          wx.request({

            url: '',

            header: { "Content-Type": "application/x-www-form-urlencoded" },

            method: 'POST',

            data: {

              openid: that.data.all[i].openid

            },

            success: function (res) {

              let p = that.data.p;

              let n = that.data.n;



              p.push(res.data.userbyid.avatarUrl)

              n.push(res.data.userbyid.nickName)



              that.setData({

                p,

                n

              })

              console.log(that.data.p)



            }



          })





        }

        // console.log(that.data.p)

        that.setData({

          stus: that.data.stu

        })







      },

    })*/





  },





  /**

   * 生命周期函数--监听页面初次渲染完成

   */

  onReady: function () {


  },



  /**

   * 生命周期函数--监听页面显示

   */

  onShow: function () {

    var that = this


    wx.request({

      url: '',

      method: 'POST',

      header: { "Content-Type": 'application/json' },



      success: function (res) {



        that.setData({

          all: res.data.circleall,

        })

        console.log(that.data.all.length)

        that.setData({

          p:[],

          n:[]

        })


        for (var i = 0; i < that.data.all.length; i++) {

          that.data.stu[i] = that.data.all[i];

          console.log(that.data.stu[i].openid)

          wx.request({

            url: '',

            header: { "Content-Type": "application/x-www-form-urlencoded" },

            method: 'POST',

            data: {

              openid: that.data.all[i].openid

            },

            success: function (res) {


           let p = that.data.p;

              let n = that.data.n;


              p.push(res.data.userbyid.avatarUrl)

              n.push(res.data.userbyid.nickName)

              that.setData({

                p,

                n

              })

              console.log(that.data.p)

            }

          })



        }

         console.log(that.data.p)

        that.setData({

          stus: that.data.stu

        })


      },

    })

  

  },



  /**

   * 生命周期函数--监听页面隐藏

   */

  onHide: function () {

  

  },



  /**

   * 生命周期函数--监听页面卸载

   */

  onUnload: function () {

  

  },



  /**

   * 页面相关事件处理函数--监听用户下拉动作

   */

  onPullDownRefresh: function () {

  

  },



  /**

   * 页面上拉触底事件的处理函数

   */

  onReachBottom: function () {

  

  },



  /**

   * 用户点击右上角分享

   */

  onShareAppMessage: function () {

  

  },

  likereact: function (e) {

   

    var that=this;

    console.log('try')

    console.log(e.target.dataset.id)

    var id = e.target.dataset.id-1

   

    console.log(that.data.stus[id].like)

    that.data.stus[id].like = that.data.stus[id].like ? 0 : 1;

    var l = "stus[" +id + "].like";

  that.setData({

    [l]: that.data.stus[id].like

  })

  },

  /*

  控制评论区

  */

  show: function () {

    this.setData({ flag: false })



  },

 

  showad: function () {

    wx.showToast({

      title: '广告位招标！',

    })



  },

  to_message:function(){

    wx.navigateTo({

      url: '../friend/my_message',

    })

  },

  liuyan:function(e){

    var that = this;

 

    that.setData({

      content:e.detail.value,

      circleid:e.target.id

    })

    console.log(that.data.circleid)

    var t

    that.setData({

      t: that.data.content,

      textdata: ' '

    })

    that.data.comment[that.data.circleid] = that.data.t

    wx.request({

      url: '',

      method: 'POST',

      header: { "Content-Type": 'application/json' },

      success: function () {

        that.setData({

          circleid: that.data.circleid,

          openid: that.data.openid,

          comment: that.data.comment,

        })

        console.log("成功！")

      },



    })



  },

  ly_btn:function(){

    var that=this

    var t

    that.setData({

      t:that.data.content,

      textdata:' '

    })

    that.data.comment[that.data.circleid]=that.data.t

    wx.request({

      url: 'https://www.lieyanwenhua.com/circominsert',

      method: 'POST',

      header: { "Content-Type": 'application/json' },

      success: function () {

        that.setData({

          circleid: that.data.circleid,

          openid: that.data.openid,

          comment: that.data.comment,

        })

        console.log("成功！")

      },



    })

    wx.showToast({

      title: '留言成功',

    })





  }

})

wxml 这里的关注点在<说说>那块啦  

<!--pages/friend/friend.wxml-->

<!--<view class='news'>

    <image class='n_pic' src='../image/news.png'></image>

    <textarea class='new' disabled='true'>{{inform}}</textarea>

</view>-->


<view class='page'>

    <view class='ad'>  

    <view class='container'>

      <!--上方轮播图片广告宣传-->

      <view class='swiper_adv'>

        <swiper indicator-dots='true' autoplay='true' interval='3000' duration='1000' circular='true' >

          <block wx:for="{{imgUrls}}">

            <swiper-item>

              <image class='swiper_img' style='width:100%;height:280rpx;' src="{{item}}" bindtap='showad' mode="aspectFit" >

              </image>

            </swiper-item>

          </block>

        </swiper>

      </view>

    </view>

  </view>


  <!--说说-->

  <view class='stu_all' >

    <view class='stu_word'>

      <view  wx:for="{{stus}}" >

        <view class='stu_item' data-id="{{item.circleid}}">

          <view class='card'>

              <!--这是主要的文字层-->

              <view class='pic_intro'>

                <image class='stu_pic' src="{{p[item.circleid-1]}}" ></image>

                <text class='stu_name' >{{n[item.circleid-1]}}</text> 

              </view>

              <view class='stu_photoa' wx:if="{{item.image1!=null |item.image2!=null|item.image2!=null}}" >

                <image class='stu_photo' src="{{item.image1}}"></image>

                <image class='stu_photo' src="{{item.image2}}"></image>

                <image class='stu_photo' src="{{item.image3}}"></image>

              </view>

              <view class='stu_word_txt'wx:if="{{item.mess!=0}}">

                <text class='stu_txt'>{{item.mess}}</text>

              </view>

              <view class='time'>

                <text class='stu_time'>{{item.time}}</text>

              </view>

              <!--这是主要的icon层-->

              <view class='like_unlike_comment'>

                <image  data-id="{{item.circleid}}" class='stu_like' src="{{item.like?'../image/like1.png' : '../image/like0.png'}}" bindtap='likereact' ></image>

              </view>  

              <view class='comment'  wx:if="{{comment[item.circleid]!=null}}">

                <text>{{comment[item.circleid]}}</text>

              </view>

              <view>

                <textarea class='liuyan' type="text" name="liuyantext" placeholder='留个言吧~' bindblur='liuyan' id="{{item.circleid}}" value='{{textdata}}'></textarea>

              </view>

            </view>  

          </view>

        </view> 

      </view>


      <!--隐藏的评论按钮-->

     <view class='stu_button'>

        <image class='stu_btn' src='../image/leavemessage.png' bindtap='to_message'></image>

      </view>


  </view>

</view>

微信小程序数组赋值