阿里巴巴Java开发手册要点笔记 (一)

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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:

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    /**     * 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));    }
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2:java.util.Objects.deepEquals(Object, Object)方法讲解分析,源码如下:

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   /**    * 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);    }
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java.util.Arrays.deepEquals0(Object, Object)源码如下:

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   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; }
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java.util.Arrays.equals(char[], char[])源码如下:

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/**     * 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;    }
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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

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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;    }}
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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:

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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;    }}
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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:

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       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
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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 3System.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。

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        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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