一.背景

一天寫程式碼出了bug,Activity傳給fragment的arraylist經過fragment中的修改居然會改變activity的arrayList(之前傳給fragment的那個)，當時我就震驚了，顛覆了我多年對Android的認知。

一開始找到這篇部落格：http://www.cnblogs.com/duanxz/p/3566221.html  裡面說bundle是傳遞引用，但是事實不是，下面例子來證明  此處還是感謝大神

顛覆的常識

1.序列化反序列化前後物件內容一樣但是肯定不是一個物件，同理ArrayList（目測Activity和Fragment之間並沒有序列化反序列化）

2.直接列印ArrayList或者物件就可以看到他的儲存地址（事實證明這是扯淡！！！

3.直接列印物件或者arraylist，或者hashcode 只要列印結果一樣，那麼它們的引用肯定是指向一個地方。（詳情看看toString原始碼，居然錯了這麼多年，慚愧啊）

二.來個栗子

1.MainActivity傳遞資料給SecondActivity

MainActivity.java

package com.example.administrator.text0212;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    private ArrayList<User> userList = new ArrayList<>();
    private ArrayList<User> userListUseBundle = new ArrayList<>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        System.out.println("xcqw onCreate");
        setContentView(R.layout.activity_main);
        for(int i = 0;i<2;i++){
            User user = new User("大熊"+i+"號","man");
            userList.add(user);
        }

        for(int i= 0;i<3;i++){
            User user = new User("bundle"+i+"號","wonman");
            userListUseBundle.add(user);
        }
        findViewById(R.id.bt_jump).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent();
                Bundle bundle = new Bundle();
                bundle.putParcelableArrayList("userListBundle",userListUseBundle);
                intent.putParcelableArrayListExtra("userList",userList);
                intent.putExtra("bundle",bundle);
                System.out.println("xcqw MainActivity userList   "+userList);
                System.out.println("xcqw MainActivity userList hashCode  "+userList.hashCode());
                System.out.println("xcqw MainActivity userListBundle   "+userListUseBundle);
                System.out.println("xcqw MainActivity userListBundle hashCode  "+userListUseBundle.hashCode());
                startActivity(intent.setClass(MainActivity.this,SecondActivity.class));
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        System.out.println("xcqw userList.size()"+userList.size());
        System.out.println("xcqw userListUseBundle.size()"+userListUseBundle.size());
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        System.out.println("xcqw onDestory");
    }
}
 

package com.example.administrator.text0212;

import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Toast;

import java.util.ArrayList;

public class SecondActivity extends AppCompatActivity {
    private ArrayList<User> userList2 = new ArrayList<>();
    private ArrayList<User> userListBundle2 = new ArrayList<>();
    public ArrayList<User> userList;
    private ArrayList<User> userListBundle;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        Intent intent = getIntent();
        userList = intent.getParcelableArrayListExtra("userList");
        Bundle bundle = intent.getBundleExtra("bundle");
        userListBundle = bundle.getParcelableArrayList("userListBundle");
        System.out.println("xcqw SecondActivity userList   " + userList);
        System.out.println("xcqw SecondActivity userList hashCode  " + userList.hashCode());
        System.out.println("xcqw SecondActivity userListBundle   " + userListBundle);
        System.out.println("xcqw SecondActivity userListBundle hashCode  " + userListBundle.hashCode());

        userList2.addAll(userList);
        userListBundle2.addAll(userListBundle);

        System.out.println("xcqw SecondActivity userList2   " + userList);
        System.out.println("xcqw SecondActivity userList2 hashCode  " + userList.hashCode());
        System.out.println("xcqw SecondActivity userListBundle2   " + userListBundle);
        System.out.println("xcqw SecondActivity userListBundle2 hashCode  " + userListBundle.hashCode());

        if (userList2 == userList) {
            System.out.println("xcqw  前後一致1");
        } else {
            System.out.println("xcqw userList2 != userList");
            System.out.println("xcqw userList.getClass()" + userList.getClass());
            System.out.println("xcqw userList2.getClass()" + userList2.getClass());
            System.out.println("xcqw SecondActivity userList   " + userList);
            System.out.println("xcqw SecondActivity userList hashCode  " + userList.hashCode());
            System.out.println("xcqw SecondActivity userList2   " + userList);
            System.out.println("xcqw SecondActivity userList2 hashCode  " + userList.hashCode());
        }

        if (userListBundle2 == userListBundle) {
            System.out.println("xcqw 前後一致2");
        } else {
            System.out.println("xcqw userListBundle2 != userListBundle");
            System.out.println("xcqw SecondActivity userListBundle   " + userListBundle);
            System.out.println("xcqw SecondActivity userListBundle hashCode  " + userListBundle.hashCode());
            System.out.println("xcqw SecondActivity userListBundle2   " + userListBundle);
            System.out.println("xcqw SecondActivity userListBundle2 hashCode  " + userListBundle.hashCode());
        }

        if (userList.equals(userList2)) {
            System.out.println("xcqw userList.equals(userList2)");
        } else {
            System.out.println("xcqw userList不equals(userList2)");
        }

        if (userListBundle.equals(userListBundle2)) {
            System.out.println("xcqw userListBundle.equals(userListBundle2)");
        } else {
            System.out.println("xcqw userListBundle不equals(userListBundle2)");
        }

        findViewById(R.id.bt_add_fragment).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ContentFragment contentFragment = new ContentFragment();
                Bundle bundle2 = new Bundle();
                bundle2.putParcelableArrayList("userListActivity", userList);
                bundle2.putString("succ", "成功了");
                contentFragment.setArguments(bundle2);
                FragmentManager fragmentManager = getSupportFragmentManager();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
                fragmentTransaction.replace(R.id.fl_container, contentFragment);
                fragmentTransaction.commitAllowingStateLoss();
            }
        });

        findViewById(R.id.bt_get_userlist).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(SecondActivity.this, "userList"+userList.size(), Toast.LENGTH_SHORT).show();
            }
        });
    }

}
 

（1）沒有覆寫User equals和hashCode

package com.example.administrator.drawlayout;

import android.os.Parcel;
import android.os.Parcelable;

/**
 * Created by Administrator on 2017/2/12 0012.
 */
public class User implements Parcelable{

    private String name;
    private String sex;

    protected User(Parcel in) {
        name = in.readString();
        sex = in.readString();
    }

    public static final Creator<User> CREATOR = new Creator<User>() {
        @Override
        public User createFromParcel(Parcel in) {
            return new User(in);
        }

        @Override
        public User[] newArray(int size) {
            return new User[size];
        }
    };

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public User() {
    }

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel parcel, int i) {
        parcel.writeString(name);
        parcel.writeString(sex);
    }

//    @Override
//    public boolean equals(Object o) {
//        if (this == o) return true;
//        if (!(o instanceof User)) return false;
//
//        User user = (User) o;
//
//        if (name != null ? !name.equals(user.name) : user.name != null) return false;
//        return sex != null ? sex.equals(user.sex) : user.sex == null;
//
//    }
//
//    @Override
//    public int hashCode() {
//        int result = name != null ? name.hashCode() : 0;
//        result = 31 * result + (sex != null ? sex.hashCode() : 0);
//        return result;
//    }

}

列印結果：看起來很符合之前的“嘗試” hashcode不一樣，當然UserList2 != userList

MainActivity userList   [[email protected], [email protected]]
MainActivity userList hashCode  186635194
MainActivity userListBundle   [[email protected], [email protected], [email protected]]
MainActivity userListBundle hashCode  -766434790


//收到Activity傳進來的資料
SecondActivity userList   [[email protected], [email protected]]
SecondActivity userList hashCode  -839526990
SecondActivity userListBundle   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle hashCode  -51512212
//Activity中 new 出來的 userList ,userListBundle2
SecondActivity userList2   [[email protected], [email protected]]
SecondActivity userList2 hashCode  -839526990
SecondActivity userListBundle2   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle2 hashCode  -51512212


userList2 != userList
userList.getClass()class java.util.ArrayList
userList2.getClass()class java.util.ArrayList
SecondActivity userList   [[email protected], [email protected]]
SecondActivity userList hashCode  -839526990
SecondActivity userList2   [[email protected], [email protected]]
SecondActivity userList2 hashCode  -839526990


userListBundle2 != userListBundle
SecondActivity userListBundle   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle hashCode  -51512212
SecondActivity userListBundle2   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle2 hashCode  -51512212


userList.equals(userList2)
userListBundle.equals(userListBundle2)

(2)重寫equals

列印和之前一樣

(3)重寫equals和hashCode

MainActivity userList   [[email protected], [email protected]]
MainActivity userList hashCode  2136183170
MainActivity userListBundle   [[email protected], [email protected], [email protected]]
MainActivity userListBundle hashCode  928096571


SecondActivity userList   [[email protected], [email protected]]
SecondActivity userList hashCode  2136183170
SecondActivity userListBundle   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle hashCode  928096571


SecondActivity userList2   [[email protected], [email protected]]
SecondActivity userList2 hashCode  2136183170
SecondActivity userListBundle2   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle2 hashCode  928096571


userList2 != userList
userList.getClass()class java.util.ArrayList
userList2.getClass()class java.util.ArrayList
SecondActivity userList   [[email protected], [email protected]]
SecondActivity userList hashCode  2136183170
SecondActivity userList2   [[email protected], [email protected]]
SecondActivity userList2 hashCode  2136183170


userListBundle2 != userListBundle
SecondActivity userListBundle   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle hashCode  928096571
SecondActivity userListBundle2   [[email protected], [email protected], [email protected]]
SecondActivity userListBundle2 hashCode  928096571


userList.equals(userList2)
userListBundle.equals(userListBundle2)

這個再一次證明記憶體地址是否一樣跟hashcode沒有任何關係，也跟直接列印物件得到的值沒關係，即使兩者相等也不會一樣

第一個例子說明:

1.hashcode和直接列印物件跟記憶體地址（也就是引用）沒有任何關係

2.Bundle傳遞也是傳資料不是傳引用！！！

2.Activity傳遞資料到Fragment

(1)沒有覆寫equals和hashCode

Content成功了
contentFragment userArrayList   [[email protected], [email protected]]
contentFragment userArrayList hashcode  -839526990
contentFragment userArrayList.toString [[email protected], [email protected]]


((SecondActivity)getActivity()).userList == userArrayList


contentFragment otherUserList    [[email protected], [email protected]]
contentFragment otherUserList hashcode    -839526990
contentFragment otherUserList.toString    [[email protected], [email protected]]


((SecondActivity)getActivity()).userList != otherUserList
userArrayListSize2
 clear userArrayList userArrayListSize0

另外點選另外一個按鈕獲取Activity中的userList 的size發現也是0

可以看到otherUserList和userArrayList雖然列印直接列印的值和hascode的值一樣，但是並沒有什麼軟用，他們還是 != 

還有就是證明了一點就是Activity傳遞資料給fragment傳的是引用

(2)覆寫equals

所有結果和上面一樣

(3)覆寫equals和hashcode

只是列印的hashcode值變了，結果沒有變

三.總結

1.hashCode和直接列印物件的值與記憶體地址沒有任何關係

2.Activity與Fragment資料傳遞會保持物件的引用，不管是parcelable還是Serizable（但是不知道為什麼???待研究）

3.判斷物件引用是否相同目前發現有且只有一個  obj1 == obj2