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谈谈java的hashcode使用场景
hashcode是在Object就已经定义了一个方法,名叫散列码,来看看Object是怎么描述它的
/** * Returns a hash code value for the object. This method is * supported for the benefit of hash tables such as those provided by * {@link java.util.HashMap}. * <p> * The general contract of {@code hashCode} is: * <ul> * <li>Whenever it is invoked on the same object more than once during * an execution of a Java application, the {@code hashCode} method * must consistently return the same integer, provided no information * used in {@code equals} comparisons on the object is modified. * This integer need not remain consistent from one execution of an * application to another execution of the same application. * <li>If two objects are equal according to the {@code equals(Object)} * method, then calling the {@code hashCode} method on each of * the two objects must produce the same integer result. * <li>It is <em>not</em> required that if two objects are unequal * according to the {@link java.lang.Object#equals(java.lang.Object)} * method, then calling the {@code hashCode} method on each of the * two objects must produce distinct integer results. However, the * programmer should be aware that producing distinct integer results * for unequal objects may improve the performance of hash tables. * </ul> * <p> * As much as is reasonably practical, the hashCode method defined by * class {@code Object} does return distinct integers for distinct * objects. (This is typically implemented by converting the internal * address of the object into an integer, but this implementation * technique is not required by the * Java™ programming language.) * * @return a hash code value for this object. * @see java.lang.Object#equals(java.lang.Object) * @see java.lang.System#identityHashCode */ public native int hashCode();
hashcode如果没有覆盖重写,那么默认是由Object导出的对象存储地址。主要应用场景是HashMap和HashSet等等的Hash集合类里面
实例1??:String的hashCode()方法
public static void main(String[] args) { String a1=new String("张"); String a2=new String("张"); String a3=new String("吴"); HashSet<String> h1=new HashSet<>(); h1.add(a1); h1.add(a2); h1.add(a3); System.out.println(h1); }
输出结果:
我明明add了三个对象进去,为什么HashSet里面只有两个元素,这是因为String覆盖了Object的hashcode()方法,我们来看看String的hashcode()
/** * Returns a hash code for this string. The hash code for a * {@code String} object is computed as * <blockquote><pre> * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] * </pre></blockquote> * using {@code int} arithmetic, where {@code s[i]} is the * <i>i</i>th character of the string, {@code n} is the length of * the string, and {@code ^} indicates exponentiation. * (The hash value of the empty string is zero.) * * @return a hash code value for this object. */ public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { char val[] = value; for (int i = 0; i < value.length; i++) { h = 31 * h + val[i]; } hash = h; } return h; }
String的hashcode值为每个字符chat值相加。HashSet虽然添加了三个对象,但是其中两个对象的值是一样的,所以HashSet只保存两个元素。
实例2??:Object的hashCode()方法
我们将实例1??的中的String类型换成Object类型,代码如下:
public static void main(String[] args) { Object a1=new Object(); Object a2=new Object(); Object a3=new Object(); HashSet<Object> h1=new HashSet<>(); h1.add(a1); h1.add(a2); h1.add(a3); System.out.println(h1); }
输出结果:
OK,HashSet保存了三个对象,没问题,因为Object的hashcode默认就是对象的存储地址
实例3??自定义类覆盖hashCode()方法
我在这里定义了一个类,代码如下:
public class HashCodeObject { private int a; public int getA() { return a; } public void setA(int a) { this.a = a; } @Override public boolean equals(Object obj) { if(obj instanceof HashCodeObject){ return this.a==((HashCodeObject)obj).getA(); } return false; } @Override public int hashCode() { return this.a; } }
然后,再将实例2??的Object换成我们自己定义的类,如下
public static void main(String[] args) { HashCodeObject a1=new HashCodeObject(); HashCodeObject a2=new HashCodeObject(); HashCodeObject a3=new HashCodeObject(); HashSet<HashCodeObject> h1=new HashSet<>(); h1.add(a1); h1.add(a2); h1.add(a3); System.out.println(h1); }
输出结果:
注意,覆盖hashcode()方法的同时也要覆盖equal()方法,不然达不到效果,并且equal()和hashcode()定义必须要一致,如果equal()返回true,那么hashcode应该
具有相同的值。
谈谈java的hashcode使用场景
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