在Java中Object类是所有类的父类,其中有几个需要override的方法比如equals,hashCode和toString等方法。
每次写这几个方法都要做很多重复性的判断, 很多类库提供了覆写这几个方法的工具类,
Guava也提供了类似的方式。下面我们来看看Guava中这几个方法简单使用。
equals方法: equals是一个经常需要覆写的方法, 可以查看Object的equals方法注释, 对equals有几个性质的要求: 当我们要覆写的类中某些值可能为null的时候,就需要对null做很多判断和分支处理。 使用Guava的Objects.equal方法可以避免这个问题, 使得equals的方法的覆写变得更加容易, 而且可读性强,简洁优雅。 import org.junit.Test; import com.google.common.base.Objects; public class ObjectTest { @Test public void equalTest() { System.out.println(Objects.equal("a", "a")); System.out.println(Objects.equal(null, "a")); System.out.println(Objects.equal("a", null)); System.out.println(Objects.equal(null, null)); } @Test public void equalPersonTest() { System.out.println(Objects.equal(new Person("peida",23), new Person("peida",23))); Person person=new Person("peida",23); System.out.println(Objects.equal(person,person)); } } class Person { public String name; public int age; Person(String name, int age) { this.name = name; this.age = age; } } 运行输出: true false false true false true hashCode方法: 当覆写(override)了equals()方法之后,必须也覆写hashCode()方法,反之亦然。
这个方法返回一个整型值(hash code value),如果两个对象被equals()方法判断为相等,那么它们就应该拥有同样的hash
code。Object类的hashCode()方法为不同的对象返回不同的值,Object类的hashCode值表示的是对象的地址。
import org.junit.Test; import com.google.common.base.Objects; public class ObjectTest { @Test public void hashcodeTest() { System.out.println(Objects.hashCode("a")); System.out.println(Objects.hashCode("a")); System.out.println(Objects.hashCode("a","b")); System.out.println(Objects.hashCode("b","a")); System.out.println(Objects.hashCode("a","b","c")); Person person=new Person("peida",23); System.out.println(Objects.hashCode(person)); System.out.println(Objects.hashCode(person)); } } class Person { public String name; public int age; Person(String name, int age) { this.name = name; this.age = age; } } 128 4066 4096 126145 19313256 19313256 toString()方法: 因为每个类都直接或间接地继承自Object,因此每个类都有toString()方法。这个方法是用得最多的, 覆写得最多, 一个好的toString方法对于调试来说是非常重要的, 但是写起来确实很不爽。Guava也提供了toString()方法。
import org.junit.Test; import com.google.common.base.Objects; public class ObjectTest { @Test public void toStringTest() { System.out.println(Objects.toStringHelper(this).add("x", 1).toString()); System.out.println(Objects.toStringHelper(Person.class).add("x", 1).toString()); Person person=new Person("peida",23); String result = Objects.toStringHelper(Person.class) .add("name", person.name) .add("age", person.age).toString(); System.out.print(result); } } class Person { public String name; public int age; Person(String name, int age) { this.name = name; this.age = age; } } //============输出=============== ObjectTest{x=1} Person{x=1} Person{name=peida, age=23} compare/compareTo方法: CompareTo:compareTo(Object
o)方法是java.lang.Comparable<T>接口中的方法,当需要对某个类的对象进行排序时,该类需要实现
Comparable<T>接口的,必须重写public int compareTo(T
o)方法。java规定,若a,b是两个对象,当a.compareTo(b)>0时,则a大于b,a.compareTo(b)<0
时,a<b,即规定对象的比较大小的规则; compareTo方法的通用约定与equals类似:将本对象与指定的对象停止比拟,如果本对象小于、等于、或大于指定对象,则分离返回正数、零、或正数。如果指定的对象类型无法与本对象停止比拟,则跑出ClassCastException。 下面我们简单自己实现一个类的compareTo方法: import org.junit.Test; public class ObjectTest { @Test public void compareTest(){ Person person=new Person("peida",23); Person person1=new Person("aida",25); Person person2=new Person("aida",25); Person person3=new Person("aida",26); Person person4=new Person("peida",26); System.out.println(person.compareTo(person1)); System.out.println(person1.compareTo(person2)); System.out.println(person1.compareTo(person3)); System.out.println(person.compareTo(person4)); System.out.println(person4.compareTo(person)); } } class Person implements Comparable<Person>{ public String name; public int age; Person(String name, int age) { this.name = name; this.age = age; } @Override public int compareTo(Person other) { int cmpName = name.compareTo(other.name); if (cmpName != 0) { return cmpName; } if(age>other.age){ return 1; } else if(age<other.age){ return -1; } return 0; } } //========输出=========== 上面的compareTo方法,代码看上去并不是十分优雅,如果实体属性很多,数据类型丰富,代码可读性将会很差。在guava里, 对所有原始类型都提供了比较的工具函数来避免这个麻烦. 比如对Integer, 可以用Ints.compare()。利用guava的原始类型的compare,我们对上面的方法做一个简化,实现compare方法: class PersonComparator implements Comparator<Person> { @Override public int compare(Person p1, Person p2) { int result = p1.name.compareTo(p2.name); if (result != 0) { return result; } return Ints.compare(p1.age, p2.age); } } 上面的代码看上去简单了一点,但还是不那么优雅简单,对此, guava有一个相当聪明的解决办法, 提供了ComparisonChain: class Student implements Comparable<Student>{ public String name; public int age; public int score; Student(String name, int age,int score) { this.name = name; this.age = age; this.score=score; } @Override public int compareTo(Student other) { return ComparisonChain.start() .compare(name, other.name) .compare(age, other.age) .compare(score, other.score, Ordering.natural().nullsLast()) .result(); } } class StudentComparator implements Comparator<Student> { @Override public int compare(Student s1, Student s2) { return ComparisonChain.start() .compare(s1.name, s2.name) .compare(s1.age, s2.age) .compare(s1.score, s2.score) .result(); } } } ComparisonChain是一个lazy的比较过程, 当比较结果为0的时候, 即相等的时候, 会继续比较下去, 出现非0的情况, 就会忽略后面的比较。ComparisonChain实现的compare和compareTo在代码可读性和性能上都有很大的提高。 下面来一个综合应用实例: import java.util.Comparator; import org.junit.Test; import com.google.common.base.Objects; import com.google.common.collect.ComparisonChain; import com.google.common.collect.Ordering; public class ObjectTest { @Test public void StudentTest(){ Student student=new Student("peida",23,80); Student student1=new Student("aida",23,36); Student student2=new Student("jerry",24,90); Student student3=new Student("peida",23,80); System.out.println("==========equals==========="); System.out.println(student.equals(student2)); System.out.println(student.equals(student1)); System.out.println(student.equals(student3)); System.out.println("==========hashCode==========="); System.out.println(student.hashCode()); System.out.println(student1.hashCode()); System.out.println(student3.hashCode()); System.out.println(student2.hashCode()); System.out.println("==========toString==========="); System.out.println(student.toString()); System.out.println(student1.toString()); System.out.println(student2.toString()); System.out.println(student3.toString()); System.out.println("==========compareTo==========="); System.out.println(student.compareTo(student1)); System.out.println(student.compareTo(student2)); System.out.println(student2.compareTo(student1)); System.out.println(student2.compareTo(student)); } } class Student implements Comparable<Student>{ public String name; public int age; public int score; Student(String name, int age,int score) { this.name = name; this.age = age; this.score=score; } @Override public int hashCode() { return Objects.hashCode(name, age); } @Override public boolean equals(Object obj) { if (obj instanceof Student) { Student that = (Student) obj; return Objects.equal(name, that.name) && Objects.equal(age, that.age) && Objects.equal(score, that.score); } return false; } @Override public String toString() { return Objects.toStringHelper(this) .addValue(name) .addValue(age) .addValue(score) .toString(); } @Override public int compareTo(Student other) { return ComparisonChain.start() .compare(name, other.name) .compare(age, other.age) .compare(score, other.score, Ordering.natural().nullsLast()) .result(); } } class StudentComparator implements Comparator<Student> { @Override public int compare(Student s1, Student s2) { return ComparisonChain.start() .compare(s1.name, s2.name) .compare(s1.age, s2.age) .compare(s1.score, s2.score) .result(); } } //=============运行输出=========================== ==========equals=========== false false true ==========hashCode=========== -991998617 92809683 -991998617 -1163491205 ==========toString=========== Student{peida, 23, 80} Student{aida, 23, 36} Student{jerry, 24, 90} Student{peida, 23, 80} ==========compareTo=========== 1 1 1 -1 |
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来自: 贾朋亮博客 > 《Guava学习笔记》