重寫hashcode()和equals()方法

• equals默認是比較內存地址，所以我們有時候需要重寫方法

• 重寫equals一定要重寫hashcode()方法！
  
  判斷重複的依據是：equals-true，hashCode-true一致性。

默認情況

public class Student {
	private String name;
	private int age;
	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	
}

public class StudentTest {
	public static void main(String[] args) {
		Student s1=new Student("aa",13);
		Student s2=new Student("aa",13);
		System.out.println(s1.equals(s2));//false,默認比較的是內存地址
	}
}

重寫方法，自定義規則

public class Student {
	private String name;
	private int age;

	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}

	public String getName() {
		return name;
	}

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	@Override
	public int hashCode() {
		return Objects.hash(this.name, this.age);
	}

	@Override
	public boolean equals(Object obj) {
		if(obj==null)
			return false;
		if(!(obj instanceof Student))
			return false;
		Student s2=(Student) obj;
		return this.name.equals(s2.name)&&this.age==s2.age;
	}

}

public class StudentTest {
	public static void main(String[] args) {
		Student s1=new Student("aa",13);
		Student s2=new Student("aa",13);
		System.out.println(s1.equals(s2));//true,重寫了equals方法
	}
}

例子：

public class Kind {
	private String kindname;
	private String discription;
	private int version;
	public String getKindname() {
		return kindname;
	}
	public void setKindname(String kindname) {
		this.kindname = kindname;
	}
	public String getDiscription() {
		return discription;
	}
	public void setDiscription(String discription) {
		this.discription = discription;
	}
	public int getVersion() {
		return version;
	}
	public void setVersion(int version) {
		this.version = version;
	}
	public Kind(String kindname, String discription, int version) {
		super();
		this.kindname = kindname;
		this.discription = discription;
		this.version = version;
	}
	
	@Override
	public String toString() {
		return "Kind [kindname=" + kindname + ", discription=" + discription + ", version=" + version + "]";
	}
	@Override
	public int hashCode() {
		String str1=this.kindname.replaceAll(" ", "").toLowerCase();
		String str2=(this.version+"").substring(0, 2);
		return Objects.hash(str1,str2);
	}

	@Override
	public boolean equals(Object obj) {
		if(obj==null)
			return false;
		if(!(obj instanceof Kind))
			return false;
		Kind another=(Kind) obj;
		if(this.kindname==null && another!=null)
			return false;
		if(this.kindname!=null&& another==null)
			return false;
		boolean result=	this.kindname.replaceAll(" ", "").equalsIgnoreCase(another.kindname.replaceAll(" ", ""));
		if(!result)
			return false;
		return (this.version+"").substring(0, 2).equals((another.version+"").substring(0, 2));
	}
	
}