Java VS Scala（一）是否相等 -

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Java VS Scala（一）是否相等

scala java

最近学习了一下scala，对scala做了一些学习后总结了一些心得，跟大家分享一下：首先，scala基于java，是一种JVM语言，跟java一样，都是通过编译器编译成class文件，由java解释器解析执行，其核心与java一样，都是运行在java虚拟机上。从语法和特性层面，scala除了具备java的面向对象基本特性（继承、封装、多态）以外，增加了函数式编程特性，而且scala语法比java更灵活，scala编程效率要比java高很多，特别是增加了许多数据操作的函数，用起来非常爽，另外scala对java的一些数据结构进行了优化提升，期性能提升不少。同样scala程序灵活易用的同时其可读性就大大降低，特别是一个初学scala的新鸟看一个scala老鸟程序的时候感觉无从下手（这点本人深有体会），当然关于这一块的讨论，网上众说纷纭，我们就不再多扯了，下面是我个人总结的一些java和scala特点对比

下面通过以下几点深入对比一下Java和Scala都有哪些异同点

1.是否相等
写过java代码的同学都知道，java对象比较有“==”和equals两种方法，“==”符号用于对象引用地址比较，equals方法默认实现与“==”相等，如果想要对比两个对象的值必须重写Object的equals方法，java的String类重写了equals方法(当然String在使用的时候也有很多注意事项，后续在做扩展)。

package com.eoi.test.scala.equals;

public class JavaEquals {

    public static void main(String args[]) {
        String aaa = "aaa";
        String bbb = "aaa";
        String ccc = new String("aaa");

        System.out.println(aaa == bbb);
        System.out.println(aaa.equals(bbb));
        System.out.println(aaa == ccc);
        System.out.println(aaa.equals(ccc));

        UserBean bean1 = new UserBean("jack", 20);
        UserBean bean2 = new UserBean("jack", 20);

        System.out.println(bean1.equals(bean2));
        System.out.println(bean1 == bean2);

        EmptyBean emptyBean1 = new EmptyBean();
        EmptyBean emptyBean2 = new EmptyBean();

        System.out.println(emptyBean1.equals(emptyBean2));
        System.out.println(emptyBean1 == emptyBean2);
    }
}

class EmptyBean {
}

class UserBean {

    private String name;
    private int age;

    UserBean(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof UserBean) {
            UserBean bean = (UserBean) obj;
            return name.equals(bean.name) && age == bean.age;
        }
        return false;
    }
}

运行结果：
true
true
false
true
true
false
false
false

scala语言里“==”和equals都是用于对象值比较，scala新增了“eq”和“ne”两个方法，比较两个对象的引用地址是否相同，例如代码：

case class User(name:String, age: Int)
class User2(name:String, age: Int)

object ScalaEquals {

  def main(args: Array[String]): Unit = {
    val aaa = "bbb"
    val bbb = "bbb"
    var ccc = "ccc1"
    var ddd = "ccc2"

    println("=============String compare=========")
    println(aaa == bbb)
    println(aaa.equals(bbb))
    println(aaa.eq(bbb))
    println(ccc == ddd)
    println(ccc.equals(ddd))
    println(ccc.eq(ddd))

    ddd = "ccc1"
    println("=============compare after value changed =========")
    println(ccc == ddd)
    println(ccc.equals(ddd))
    println(ccc.eq(ddd))

    val eee = new String("111")
    val fff = new String("111")

    var ggg = new String("222")
    var hhh = new String("222")

    println("============= compare new String ========")
    println(eee == fff)
    println(eee.equals(fff))
    println(eee.eq(fff))

    println(ggg == hhh)
    println(ggg.equals(hhh))
    println(ggg.eq(hhh))

    val jack = new User("jack", 21)
    val jack2 = new User("jack", 21)
    val jack3 = new User("jack", 20)
    val jack4 = jack

    println("==========cass class compare============")
    println(jack == jack2)
    println(jack.equals(jack2))
    println(jack.eq(jack2))

    println(jack == jack3)
    println(jack.equals(jack3))
    println(jack.eq(jack3))

    println(jack == jack4)
    println(jack.equals(jack4))
    println(jack.eq(jack4))

    val jack5 = new User2("jack", 21)
    val jack6 = new User2("jack", 21)
    val jack7 = new User2("jack", 20)
    val jack8 = jack5

    println("============class compare============")
    println(jack5 == jack6)
    println(jack5.equals(jack6))
    println(jack5.eq(jack6))

    println(jack5 == jack7)
    println(jack5.equals(jack7))
    println(jack5.eq(jack7))

    println(jack8 == jack6)
    println(jack8.equals(jack6))
    println(jack8.eq(jack6))

  }
}

运行结果：
=============String compare=========
true
true
true
false
false
false
=============compare after value changed =========
true
true
true
============= compare new String ========
true
true
false
true
true
false
==========cass class compare============
true
true
false
false
false
false
true
true
true
============class compare============
false
false
false
false
false
false
false
false
false

从上面的例子中可以看出，User和User2的对比结果完全不一样，因为一个是case class 一个是 class，透过现象看本质，我们两个类的class文件反编译以后可以看到，User2

import scala.reflect.ScalaSignature;

public class User2
{
  public User2(String name, int age) {}
}

User编译后生成两个class文件，User.class和User$.class，User.class里面重写了equals、hashCode等方法，User.class方法的实际调用时User$.class里的MODULE$

package com.eoi.test.scala.equals;

import scala.Function1;
import scala.Option;
import scala.Product;
import scala.Product.class;
import scala.Serializable;
import scala.Tuple2;
import scala.collection.Iterator;
import scala.reflect.ScalaSignature;
import scala.runtime.BoxesRunTime;
import scala.runtime.ScalaRunTime.;
import scala.runtime.Statics;

public class User
  implements Product, Serializable
{
  private final String name;
  
  public User(String name, int age)
  {
    Product.class.$init$(this);
  }
  
  /* Error */
  public boolean equals(Object x$1)
  {
    // Byte code:
    //   0: aload_0
    //   1: aload_1
    //   2: if_acmpeq +90 -> 92
    //   5: aload_1
    //   6: astore_2
    //   7: aload_2
    //   8: instanceof 2
    //   11: ifeq +8 -> 19
    //   14: iconst_1
    //   15: istore_3
    //   16: goto +5 -> 21
    //   19: iconst_0
    //   20: istore_3
    //   21: iload_3
    //   22: ifeq +74 -> 96
    //   25: aload_1
    //   26: checkcast 2	com/eoi/test/scala/equals/User
    //   29: astore 4
    //   31: aload_0
    //   32: invokevirtual 53	com/eoi/test/scala/equals/User:name	()Ljava/lang/String;
    //   35: aload 4
    //   37: invokevirtual 53	com/eoi/test/scala/equals/User:name	()Ljava/lang/String;
    //   40: astore 5
    //   42: dup
    //   43: ifnonnull +12 -> 55
    //   46: pop
    //   47: aload 5
    //   49: ifnull +14 -> 63
    //   52: goto +36 -> 88
    //   55: aload 5
    //   57: invokevirtual 113	java/lang/Object:equals	(Ljava/lang/Object;)Z
    //   60: ifeq +28 -> 88
    //   63: aload_0
    //   64: invokevirtual 56	com/eoi/test/scala/equals/User:age	()I
    //   67: aload 4
    //   69: invokevirtual 56	com/eoi/test/scala/equals/User:age	()I
    //   72: if_icmpne +16 -> 88
    //   75: aload 4
    //   77: aload_0
    //   78: invokevirtual 115	com/eoi/test/scala/equals/User:canEqual	(Ljava/lang/Object;)Z
    //   81: ifeq +7 -> 88
    //   84: iconst_1
    //   85: goto +4 -> 89
    //   88: iconst_0
    //   89: ifeq +7 -> 96
    //   92: iconst_1
    //   93: goto +4 -> 97
    //   96: iconst_0
    //   97: ireturn
    // Line number table:
    //   Java source line #3	-> byte code offset #0
    // Local variable table:
    //   start	length	slot	name	signature
    //   0	98	0	this	User
    //   0	98	1	x$1	Object
  }
  
  public String toString()
  {
    return ScalaRunTime..MODULE$._toString(this);
  }
  
  public int hashCode()
  {
    int i = -889275714;i = Statics.mix(i, Statics.anyHash(name()));i = Statics.mix(i, age());return Statics.finalizeHash(i, 2);
  }
  
  public boolean canEqual(Object x$1)
  {
    return x$1 instanceof User;
  }
  
  public Iterator<Object> productIterator()
  {
    return ScalaRunTime..MODULE$.typedProductIterator(this);
  }
  
  public Object productElement(int x$1)
  {
    int i = x$1;
    switch (i)
    {
    default: 
      throw new IndexOutOfBoundsException(BoxesRunTime.boxToInteger(x$1).toString());
    case 1: 
      break;
    }
    return name();
  }
  
  public int productArity()
  {
    return 2;
  }
  
  public String productPrefix()
  {
    return "User";
  }
  
  public int copy$default$2()
  {
    return age();
  }
  
  public String copy$default$1()
  {
    return name();
  }
  
  public User copy(String name, int age)
  {
    return new User(name, age);
  }
  
  public int age()
  {
    return this.age;
  }
  
  public String name()
  {
    return this.name;
  }
  
  public static Function1<String, Function1<Object, User>> curried()
  {
    return User..MODULE$.curried();
  }
  
  public static Function1<Tuple2<String, Object>, User> tupled()
  {
    return User..MODULE$.tupled();
  }
  
  public static User apply(String paramString, int paramInt)
  {
    return User..MODULE$.apply(paramString, paramInt);
  }
  
  public static Option<Tuple2<String, Object>> unapply(User paramUser)
  {
    return User..MODULE$.unapply(paramUser);
  }
}

package com.eoi.test.scala.equals;

import scala.None.;
import scala.Option;
import scala.Serializable;
import scala.Some;
import scala.Tuple2;
import scala.runtime.AbstractFunction2;
import scala.runtime.BoxesRunTime;

public final class User$
  extends AbstractFunction2<String, Object, User>
  implements Serializable
{
  public static final  MODULE$;
  
  private User$()
  {
    MODULE$ = this;
  }
  
  private Object readResolve()
  {
    return MODULE$;
  }
  
  public Option<Tuple2<String, Object>> unapply(User x$0)
  {
    return x$0 == null ? None..MODULE$ : new Some(new Tuple2(x$0.name(), BoxesRunTime.boxToInteger(x$0.age())));
  }
  
  public User apply(String name, int age)
  {
    return new User(name, age);
  }
  
  public final String toString()
  {
    return "User";
  }
  
  static
  {
    new ();
  }
}

总结，Scala中的case class自动重写了equals和hashCode方法，所以实现了直接的值对比