Java中 ==和equals()方法的區別
阿新 • • 發佈:2021-01-31
1、==
==既可以比較基本資料型別,也可以比較引用資料型別。對於基本資料比較的是值,對於引用資料型別比較的是記憶體地址。
class A{
int i;
}
public class EqualTest {
@Test
public void test1(){
int i = 10;
double j = 10.0;
if (i == j){
System.out.println("相等");
}else{
System. out.println("不相等");
}
/*結果:
相等
*/
}
@Test
public void test2(){
A a1 = new A();
A a2 = new A();
if (a1 == a2){
System.out.println("相等");
}else{
System.out.println(a1);
System. out.println(a2);
System.out.println("不相等");
}
/*結果:
[email protected]
[email protected]
不相等
*/
}
}
2、equals()
equals()方法只能比較引用資料型別。在比較的時候,會出現以下幾種情況:
1、如果類中沒有重寫equals方法,則比較的是物件的型別和地址:
class A{
int i;
}
public class EqualTest {
@Test
public void test3(){
A a1 = new A();
A a2 = new A();
a1.i = a2.i = 3;
if (a1.equals(a2)){
System.out.println(a1 + "\n" + a2);
System.out.println("相等");
}else{
System.out.println(a1 + "\n" + a2);
System.out.println("不相等");
}
/*
[email protected]
[email protected]
不相等
*/
}
}
2、如果類中重寫了equals方法,則比較的是物件的型別和值:
class A{
int i;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
A a = (A) o;
return i == a.i;
}
}
public class EqualTest {
@Test
public void test3(){
A a1 = new A();
A a2 = new A();
a1.i = a2.i = 3;
if (a1.equals(a2)){
System.out.println(a1 + "\n" + a2);
System.out.println("相等");
}else{
System.out.println(a1 + "\n" + a2);
System.out.println("不相等");
}
/*
[email protected]
[email protected]
相等
*/
}
}
為什麼說是型別和地址呢 ?下面的這個例子給出解釋:
class A{
int i;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
A a = (A) o;
return i == a.i;
}
}
class B{
int i;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
B b = (B) o;
return i == b.i;
}
}
public class EqualTest {
@Test
public void test3(){
A a1 = new A();
B b1 = new B();
a1.i = b1.i = 3;
if (a1.equals(b1)){
System.out.println(a1 + "\n" + b1);
System.out.println("相等");
}else{
System.out.println(a1 + "\n" + b1);
System.out.println("不相等");
}
/*
[email protected]
[email protected]
不相等
*/
}
}