Kotlin与Java 代码比对学习

User.java

package com.example.app.entity;

public class User {
    private String username;
    private String password;
    private String code;

    public User() {

    }

    public User(String username, String password, String code) {
        this.username = username;
        this.password = password;
        this.code = code;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

}

User.kt

class User {

    //kotlin中默认有get set方法
    var username: String? = null
    // ?表示可以赋值为null
    var password: String? = null
    var code: String? = null

    //空构造函数
    constructor() {

    }

    //带参数的构造函数
    constructor(username: String?, password: String?, code: String?) {
        this.username = username
        this.password = password
        this.code = code
    }
}

MainActivity.java

package com.example.app;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import com.example.app.entity.User;
import com.example.app.widget.CodeView;
import com.example.core.utils.CacheUtils;
import com.example.core.utils.Utils;
import com.example.lesson.LessonActivity;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity
        implements View.OnClickListener {

    private final String usernameKey = "username";
    private final String passwordKey = "password";

    private EditText et_username;
    private EditText et_password;
    private EditText et_code;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        et_username = findViewById(R.id.et_username);
        et_password = findViewById(R.id.et_password);
        et_code = findViewById(R.id.et_code);

        et_username.setText(CacheUtils.get(usernameKey));
        et_password.setText(CacheUtils.get(passwordKey));

        final Button btn_login = findViewById(R.id.btn_login);
        final CodeView img_code = findViewById(R.id.code_view);
        btn_login.setOnClickListener(this);
        img_code.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        if (v instanceof CodeView) {
            CodeView codeView = (CodeView) v;
            codeView.updateCode();
        } else if (v instanceof Button) {
            login();
        }
    }

    private void login() {
        final String username = et_username.getText().toString();
        final String password = et_password.getText().toString();
        final String code = et_code.getText().toString();

        final User user = new User(username, password, code);
        if (verify(user)) {
            CacheUtils.save(usernameKey, username);
            CacheUtils.save(passwordKey, password);
            startActivity(new Intent(this, LessonActivity.class));
        }
    }

    private boolean verify(User user) {
        if (user.getUsername() != null && user.getUsername().length() < 4) {
            Utils.toast("用户名不合法");
            return false;
        }
        if (user.getPassword() != null && user.getPassword().length() < 4) {
            Utils.toast("密码不合法");
            return false;
        }
        return true;
    }
}

MainActivity.kt

package com.example.app.activity

import android.content.Intent
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.EditText
import androidx.appcompat.app.AppCompatActivity
import com.kotlin.app.R
import com.example.app.entity.User
import com.example.app.widget.CodeView
import com.example.core.utils.CacheUtils
import com.example.core.utils.toast
import com.example.lesson.LessonActivity

//:可以代表集成，如果有实现的接口，后面用逗号分隔
class MainActivity : AppCompatActivity(), View.OnClickListener {

    private val usernameKey = "username"
    private val passwordKey = "password"

    //类型后面问好，代表可以为null，kotlin定义变量必须有初始值
    //private var et_username:EditText? = null

    //lateinit 表示延时初始化，可以先不给初始值
    private lateinit var et_username: EditText
    private lateinit var et_password: EditText
    //Kotlin中代码结尾不用写封号
    private lateinit var et_code: EditText

    private lateinit var btn_login: Button
    private lateinit var img_code: CodeView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        //初始化view
        initView()
        //点击事件
        clickEvent()
    }



    private fun initView() {
        et_username = findViewById<EditText>(R.id.et_username)
        et_password = findViewById<EditText>(R.id.et_password)
        et_code = findViewById<EditText>(R.id.et_code)
        btn_login = findViewById<Button>(R.id.btn_login)
        img_code = findViewById<CodeView>(R.id.code_view)

    }

    private fun clickEvent() {
        //?. 表示 btn_login如果不为null,调用点击事件
        //!!:不会校验null值，无论是否为null，都会调用
        btn_login?.setOnClickListener(this)
        img_code?.setOnClickListener(this)

    }

    override fun onClick(v: View) {
        //Kotlin中 类型判断用is
        if(v is CodeView){
            v.updateCode()
        }else if(v is Button){
            login()
        }
    }

    private fun login() {
        //Java 中的final 表示不可变引用 getText kotlin中直接.text
        val username:String = et_username.text.toString()
        val password:String= et_password.text.toString()
        val code:String= et_code.text.toString()

        //Kotlin 中创建对象不用new
        val user: User = User(username,password,code)

        if(verify(user)){
            CacheUtils.save(usernameKey, username)
            CacheUtils.save(passwordKey, password)
            //Kotlin中 LessonActivity.class 要写成LessonActivity::class.java
            //Kotlin中获取class 用LessonActivity::class 要转化成java中class-->LessonActivity::class.java
            startActivity(Intent(this, LessonActivity::class.java))
        }
    }

    private fun verify(user: User): Boolean {
        if(user.username!=null && user.username!!.length<4){
            toast("用户名不合法")
            return false
        }
        if (user.password != null && user.password!!.length < 4) {
           toast("密码不合法")
            return false
        }
        return true
    }
}

Java中的数组

private String[] codeList = new String[]{"kotlin", "android", "java", "http", "https", "okhttp", "retrofit", "tcp/ip"};

Kotlin中的数组

 //Kotlin 中数组用 arrayOf
 private val codeList =arrayOf("kotlin", "android", "java", "http", "https", "okhttp", "retrofit", "tcp/ip")

Java中构造函数：

    public CodeView(Context context) {
        this(context, null);
    }

    public CodeView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
       
    }

Kotlin中构造函数：

    //:this(context,null) 调用双参构造
    constructor(context: Context) : this(context, null)

    //:super(context,attrs) 调用父类构造函数
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
       
    }

Kotlin中写静态函数或者静态成员的三种方式：

顶层函数

在java中写一个工具类是这样的

public class Utils {

    private static final DisplayMetrics displayMetrics = Resources.getSystem().getDisplayMetrics();

    public static float dp2px(float dp) {
        return TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, displayMetrics);
    }

    public static void toast(String string) {
        toast(string, Toast.LENGTH_SHORT);
    }

    public static void toast(String string, int duration) {
        Toast.makeText(BaseApplication.currentApplication(), string, duration).show();
    }

}

用Kotlin的顶层函数是这样的 

 Utils.kt

@file:JvmName("MyUtils")
package com.example.core.utils

import android.content.res.Resources
import android.util.DisplayMetrics
import android.util.TypedValue
import android.widget.Toast
import com.example.core.BaseApplication

//Kotlin中写静态函数，有三种方法
//第一种方法：顶层函数（Kotlin中的属性和方法可以直接写在文件中，就是类的外层）

private val displayMetrics: DisplayMetrics = Resources.getSystem().getDisplayMetrics()


fun dp2px(dp: Float): Float {
    return TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, displayMetrics)
}

fun toast(string: String) {
    toast(string, Toast.LENGTH_SHORT)
}

fun toast(string: String, duration: Int) {
    Toast.makeText(BaseApplication.currentApplication(), string, duration).show()
}

在kotlin中直接通过包名调用

在java代码中是这样调用的

//在Java代码中调用Kotlin的顶层函数，如果没有使用注解：@file:JvmName("MyUtils")，需要在工具类后面加上Kt字符
//也可以通过注解自定义
  MyUtils.dp2px(20f);

object类

在java中的一个工具类是这样的

package com.example.core.utils;

import android.annotation.SuppressLint;
import android.content.Context;
import android.content.SharedPreferences;

import com.example.core.BaseApplication;
import com.example.core.R;

public class CacheUtils {
    @SuppressLint("StaticFieldLeak")
    private static Context context = BaseApplication.currentApplication();

    private static SharedPreferences SP = context.getSharedPreferences(context.getString(R.string.app_name), Context.MODE_PRIVATE);

    public static void save(String key, String value) {
        SP.edit().putString(key, value).apply();
    }

    public static String get(String key) {
        return SP.getString(key, null);
    }
}

kotlin中可以这样写：

package com.example.core.utils

import android.content.Context
import android.content.SharedPreferences
import com.example.core.BaseApplication
import com.example.core.R


//在Kotlin中，如果一个类被object 修饰。就表示这个类中的所有的成员变量
//和成员函数都是静态的。也就是一个单列类
object CacheUtils {
    val context: Context = BaseApplication.currentApplication()

    val SP: SharedPreferences =
        context.getSharedPreferences(context.getString(R.string.app_name), Context.MODE_PRIVATE)

    //Unit代表没有
    fun save(key: String?, value: String?): Unit {
        SP.edit().putString(key, value)
    }

    //String? 返回值后面加上一个问号，代表返回值可能为null
    @JvmStatic
    fun get(key: String?): String? {
        return SP.getString(key, null)
    }

}

在kotlin中直接调用  

CacheUtils.save(key, username)

在Java代码中调用kotlin中object修饰的类,如果不用@JvmStatic注解必须用.INSTANCE

CacheUtils.get("Kotlin学习");或者 CacheUtils.INSTANCE.get("Kotlin学习");

companion object 伴生对象

在java中一个类是这样的

package com.example.core;

import android.app.Application;
import android.content.Context;

import androidx.annotation.NonNull;

public class BaseApplication extends Application {

    private static Context currentApplication;

    @NonNull
    public static Context currentApplication() {
        return currentApplication;
    }

    @Override
    protected void attachBaseContext(Context base) {
        super.attachBaseContext(base);
        currentApplication = this;
    }
}

Kotlin中就是这样的：

package com.example.core

import android.app.Application
import android.content.Context

class BaseApplication : Application() {

    //object 修饰的对象中的变量和函数都是静态的，
    //我们只想让类中的一部分函数和变量是静态的就用伴生对象

    companion object {
        lateinit var currentApplication: Context

        @JvmStatic
        fun currentApplication(): Context {
            return currentApplication
        }
    }

    override fun attachBaseContext(base: Context) {
        super.attachBaseContext(base)
        currentApplication = this
    }

}

在kotlin中直接类名就可以调用：

BaseApplication.currentApplication()

在java中调用这个kotlin代码

 //在Java中调用伴生对象  如果不用@JvmStatic注解必须用.Companion
 //BaseApplication.Companion.currentApplication();
   BaseApplication.currentApplication();

java中的接口定义与Kotlin中的接口定义

java中的接口

public interface EntityCallback<T> {
    void onSuccess(@NonNull T entity);

    void onFailure(@Nullable String message);
}

kotlin中的接口

//Kotlin中接口

interface EntityCallback<T>{
    fun onSuccess(entity:T)
    fun onFailure(message:String?)
}

Java中抽象类与kotlin中的抽象类

java中的抽象类

public abstract class BaseViewHolder extends RecyclerView.ViewHolder {
    public BaseViewHolder(@NonNull View itemView) {
        super(itemView);
    }

    @SuppressLint("UseSparseArrays")
    private final Map<Integer, View> viewHashMap = new HashMap<>();

    @SuppressWarnings("unchecked")
    protected <T extends View> T getView(@IdRes int id) {
        View view = viewHashMap.get(id);
        if (view == null) {
            view = itemView.findViewById(id);
            viewHashMap.put(id, view);
        }
        return (T) view;
    }

    protected void setText(@IdRes int id, @Nullable String text) {
        ((TextView) getView(id)).setText(text);
    }
}

Kotlin中的抽象类

abstract class BaseViewHolder: RecyclerView.ViewHolder{
    //构造函数

    constructor(itemView: View):super(itemView)

    private val viewHashMap:HashMap<Int,View> = HashMap<Int,View>()
    
    fun<T:View> getView(id:Int):T{
        var view:View? = viewHashMap.get(id)
        if(view==null){
            view = itemView.findViewById(id)
            viewHashMap.put(id, view)
        }
        //kotlin 中类型转换用as
        return view as T
    }

    protected fun setText(@IdRes id: Int, text: String?) {
        (getView<View>(id) as TextView).text = text
    }
}

Java中的枚举与Kotlin中的枚举

Java中枚举 enum

public enum StateMessage {

}

Kotlin中枚举 enum class

enum class StateMessage {

}

Java、Kotlin中的接口回调

Java中的接口回调

 call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                entityCallback.onFailure("网络异常");
            }

            @Override
            public void onResponse(Call call, Response response) {
                final int code = response.code();
                if (code >= 200 && code < 300) {
                    final ResponseBody body = response.body();
                    String json = null;
                    try {
                        json = body.string();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    entityCallback.onSuccess((T) convert(json, type));
                } else if (code >= 400 && code < 500) {
                    entityCallback.onFailure("客户端错误");
                } else if (code > 500 && code < 600) {
                    entityCallback.onFailure("服务器错误");
                } else {
                    entityCallback.onFailure("未知错误");
                }
            }
        });

Kotlin中的接口回调

        //Kotlin中回调函数 用object:
        call.enqueue(object : Callback {
            override fun onFailure(call: Call, e: IOException) {
                entityCallback.onFailure("网络异常")
            }

            override fun onResponse(call: Call, response: Response) {
                val code: Int = response.code()
                when (code) {
                    in 200..299 -> {
                        val body: ResponseBody? = response.body()
                        var json: String? = null
                        json = body?.string()
                        entityCallback.onSuccess(convert(json, type) as T)
                    }
                    in 400..499 -> { entityCallback.onFailure("客户端错误") }
                    in 500..599 -> { entityCallback.onFailure("服务器错误") }
                    else -> entityCallback.onFailure("未知错误")
                }
            }
        })