Object 的 equals 方法容易抛空指针异常，应使用常量或确定有值的对象来调用 equals。
正例："test".equals(object);
反例：object.equals("test");
- 推荐使用 java.util.Objects#equals（JDK7 引入的工具类）。其就是实例对象equals的一个健壮版本
实现：
/**
* Returns {@code true} if the arguments are equal to each other
* and {@code false} otherwise.
* Consequently, if both arguments are {@code null}, {@code true}
* is returned and if exactly one argument is {@code null}, {@code
* false} is returned. Otherwise, equality is determined by using
* the {@link Object#equals equals} method of the first
* argument.
*
* @param a an object
* @param b an object to be compared with {@code a} for equality
* @return {@code true} if the arguments are equal to each other
* and {@code false} otherwise
* @see Object#equals(Object)
*/
public static boolean equals(Object a, Object b) {
// 可以巧妙避开空指针异常。首先判断a==b，在判断a！=null，如果a==null的话此时a.equals(b)短路
return (a == b) || (a != null && a.equals(b));
}
- java.util.Objects.deepEquals(Object, Object)方法讲解分析，源码如下：
/**
* Returns {@code true} if the arguments are deeply equal to each other
* and {@code false} otherwise.
*
* Two {@code null} values are deeply equal. If both arguments are
* arrays, the algorithm in {@link Arrays#deepEquals(Object[],
* Object[]) Arrays.deepEquals} is used to determine equality.
* Otherwise, equality is determined by using the {@link
* Object#equals equals} method of the first argument.
*
* @param a an object
* @param b an object to be compared with {@code a} for deep equality
* @return {@code true} if the arguments are deeply equal to each other
* and {@code false} otherwise
* @see Arrays#deepEquals(Object[], Object[])
* @see Objects#equals(Object, Object)
*/
public static boolean deepEquals(Object a, Object b) {
if (a == b)
return true;
else if (a == null || b == null)
return false;
else
return Arrays.deepEquals0(a, b);
}
java.util.Arrays.deepEquals0(Object, Object)源码如下：
static boolean deepEquals0(Object e1, Object e2) {
assert e1 != null;
boolean eq;
// 如下的equals是Arrays中的equals方法，是对每一个元素进行比较
if (e1 instanceof Object[] && e2 instanceof Object[])
eq = deepEquals ((Object[]) e1, (Object[]) e2);
else if (e1 instanceof byte[] && e2 instanceof byte[])
eq = equals((byte[]) e1, (byte[]) e2);
else if (e1 instanceof short[] && e2 instanceof short[])
eq = equals((short[]) e1, (short[]) e2);
else if (e1 instanceof int[] && e2 instanceof int[])
eq = equals((int[]) e1, (int[]) e2);
else if (e1 instanceof long[] && e2 instanceof long[])
eq = equals((long[]) e1, (long[]) e2);
else if (e1 instanceof char[] && e2 instanceof char[])
eq = equals((char[]) e1, (char[]) e2);
else if (e1 instanceof float[] && e2 instanceof float[])
eq = equals((float[]) e1, (float[]) e2);
else if (e1 instanceof double[] && e2 instanceof double[])
eq = equals((double[]) e1, (double[]) e2);
else if (e1 instanceof boolean[] && e2 instanceof boolean[])
eq = equals((boolean[]) e1, (boolean[]) e2);
else
eq = e1.equals(e2);
return eq;
}
java.util.Arrays.equals(char[], char[])源码如下：
/**
* Returns <tt>true</tt> if the two specified arrays of chars are
* <i>equal</i> to one another. Two arrays are considered equal if both
* arrays contain the same number of elements, and all corresponding pairs
* of elements in the two arrays are equal. In other words, two arrays
* are equal if they contain the same elements in the same order. Also,
* two array references are considered equal if both are <tt>null</tt>.<p>
*
* @param a one array to be tested for equality
* @param a2 the other array to be tested for equality
* @return <tt>true</tt> if the two arrays are equal
*/
public static boolean equals(char[] a, char[] a2) {
if (a==a2)
return true;
if (a==null || a2==null)
return false;
int length = a.length;
if (a2.length != length)
return false;
for (int i=0; i<length; i++)
if (a[i] != a2[i])
return false;
return true;
}
示例1：对于基本类型的数组，元素相同，使用Objects的equals方法判定是false，但是对于deepEquals方法是对底层每一个元素调用其equals判断是否相等。
int[] a = { 1, 2, 3, 4, 5 };
int[] b = { 1, 2, 3, 4, 5 };
// 数组的equals方法其实就是直接判断引用是否相等，不会具体判断每一个元素
System.out.println(Objects.equals(a, b)); // false
System.out.println(Objects.deepEquals(a, b)); // true
示例2：
实体类User，此版本实现了HashCode和equals方法：
class User {
public String userName;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((userName == null) ? 0 : userName.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
User other = (User) obj;
if (userName == null) {
if (other.userName != null)
return false;
} else if (!userName.equals(other.userName))
return false;
return true;
}
public User(String userName) {
super();
this.userName = userName;
}
public User() {
super();
// TODO Auto-generated constructor stub
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
}
此时：
User[] u1 = { new User("daxin"), new User("mali") };
User[] u2 = { new User("daxin"), new User("mali") };
System.out.println(Objects.equals(u1, u2)); //false
System.out.println(Objects.deepEquals(u1, u2)); //true，调用的User自己实现的equals方法
示例3：没有实现hashcode和equals方法的User：
class User {
public String userName;
public User(String userName) {
super();
this.userName = userName;
}
public User() {
super();
// TODO Auto-generated constructor stub
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
}
此时：
User[] u1 = { new User("daxin"), new User("mali") };
User[] u2 = { new User("daxin"), new User("mali") };
System.out.println(Objects.equals(u1, u2)); //false
System.out.println(Objects.deepEquals(u1, u2)); //false，对底层每一个元素调用默认的equals方法（此时equals方法判定的是引用）
- 所有的相同类型的包装类对象之间值的比较，全部使用 equals 方法比较。 说明：对于 Integer var = ? 在-128 至 127 范围内的赋值，Integer 对象是在IntegerCache.cache 产生，会复用已有对象，这个区间内的 Integer 值可以直接使用==进行 判断，但是这个区间之外的所有数据，都会在堆上产生，并不会复用已有对象，这是一个大坑， 推荐使用 equals 方法进行判断。
Integer a=12;
Integer b=12;
System.out.println(a==b);//true
Integer c=1550;
Integer d=1550;
System.out.println(c==d);//false
System.out.println(c.intValue()==d);//对c进行拆箱，所以导致d也拆箱。所以true
System.out.println(c==d.intValue());//对d进行拆箱，所以导致c也拆箱。所以true
System.out.println(c.intValue()==d.intValue());//都拆箱
System.out.println(Objects.equals(c, d));//使用Objects的equals方法比较
- 使用索引访问用 String 的 split 方法得到的数组时，需做最后一个分隔符后有无 内容的检查，否则会有抛 IndexOutOfBoundsException 的风险。
说明：
String str = "a,b,c,,";
String[] ary = str.split(",");
// 预期大于 3，结果是 3
System.out.println(ary.length);
又如：
System.out.println(",,,,".split(",").length); //0
- ArrayList的subList结果不可强转成ArrayList，否则会抛出ClassCastException 异常，即 java.util.RandomAccessSubList cannot be cast to java.util.ArrayList. 说明：subList 返回的是 ArrayList 的内部类 SubList，并不是 ArrayList ，而是 ArrayList 的一个视图，对于 SubList 子列表的所有操作最终会反映到原列表上。
List<Integer> list =new ArrayList<>();
list.add(1);
list.add(2);
List<Integer> subList = list.subList(0, list.size());
System.out.println(subList);//[1, 2]
subList.add(3); //对subList操作反映在list上
System.out.println(list);//[1, 2, 3]
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