阿里巴巴Java开发手册要点笔记 (一)
1：【强制】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");
说明：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)); }
2：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）
3：【强制】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。
示例：
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
4：【推荐】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
5：【强制】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]
