hashCode() vs equals() vs ==
阿新 • • 發佈:2022-02-11
==
VS equals()
==
- 基礎型別:
==
比較的是值 - 引用型別:
==
比較的是物件的記憶體地址
equals()
equals()
只能比較引用型別,無法比較基礎型別.equals()
方法在頂級父類Object
中,程式碼如下:
public boolean equals(Object obj) {
return (this == obj);
}
可以看出這個程式碼就是判斷是否是同一物件.那麼,當子類重寫 equals()
往往都是將屬性內容相同的物件認為是同一物件,
如果子類不直接或間接重寫Object
的equals()
方法,那麼呼叫的equals()
==
相同.
String a = new String("aa"); // a 為一個引用
String b = new String("aa"); // b為另一個引用,物件的內容一樣
String aa = "aa"; // 放在常量池中
String bb = "aa"; // 從常量池中查詢
System.out.println(aa == bb);// true
System.out.println(a == b);// false
System.out.println(a.equals(b));// true
上面程式碼的String
重寫了equals()
,程式碼如下:
public boolean equals(Object anObject) { if (this == anObject) { return true; } if (anObject instanceof String) { String anotherString = (String)anObject; int n = value.length; if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; while (n-- != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; }
hashCode()
VS equals()
hashCode()
函式返回雜湊碼,確定該物件在雜湊表中的索引位置.同樣屬於Object
類中,程式碼如下:
public native int hashCode();
,native呼叫C/C++,返回int
雜湊碼.利用雜湊碼能夠快速檢索出物件
如HashSet
/HashMap
會先呼叫hashCode()
,如果雜湊碼不同,直接判斷物件不同,否則進行下面的equals()
操作,大大減少了equals()
的操作,提高執行速度.(C/C++本身比Java執行快,equals()執行也比hashCode()邏輯複雜)hashCode()
返回值相同也不能認為是同一物件,存在hash衝突- hashCode 相同,equals()為true才能認為是同一物件
為什麼重寫 equals()
時必須重寫 hashCode()
方法?
- 正面:兩物件相等,那麼hashCode也相等,equals()也為true
- 反面:重寫
equals()
,但是不重寫hashCode()
,會導致equals(),判斷是同一個物件,但是雜湊碼並不同 - 例子: 重寫了
equals()
方法,不重寫hashCode()
,同一名學生,新增到hashSet中時候,會新增兩次,因為你只是通過屬性內容判斷的是否為同一物件!!!
包裝型別的常量池
- Byte/Short/Integer/Long 這 4 種包裝類預設建立了數值 [-128,127] 的相應型別的快取資料
- Character 建立了數值在 [0,127] 範圍的快取資料
- Boolean 直接true/false
原始碼如下:
private static class ByteCache {
private ByteCache(){}
static final Byte cache[] = new Byte[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Byte((byte)(i - 128));
}
}
/**
* Returns a {@code Byte} instance representing the specified
* {@code byte} value.
* If a new {@code Byte} instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Byte(byte)}, as this method is likely to yield
* significantly better space and time performance since
* all byte values are cached.
*
* @param b a byte value.
* @return a {@code Byte} instance representing {@code b}.
* @since 1.5
*/
public static Byte valueOf(byte b) {
final int offset = 128;
return ByteCache.cache[(int)b + offset];
}
private static class ShortCache {
private ShortCache(){}
static final Short cache[] = new Short[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Short((short)(i - 128));
}
}
/**
* Returns a {@code Short} instance representing the specified
* {@code short} value.
* If a new {@code Short} instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Short(short)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* This method will always cache values in the range -128 to 127,
* inclusive, and may cache other values outside of this range.
*
* @param s a short value.
* @return a {@code Short} instance representing {@code s}.
* @since 1.5
*/
public static Short valueOf(short s) {
final int offset = 128;
int sAsInt = s;
if (sAsInt >= -128 && sAsInt <= 127) { // must cache
return ShortCache.cache[sAsInt + offset];
}
return new Short(s);
}
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
/**
* Returns an {@code Integer} instance representing the specified
* {@code int} value. If a new {@code Integer} instance is not
* required, this method should generally be used in preference to
* the constructor {@link #Integer(int)}, as this method is likely
* to yield significantly better space and time performance by
* caching frequently requested values.
*
* This method will always cache values in the range -128 to 127,
* inclusive, and may cache other values outside of this range.
*
* @param i an {@code int} value.
* @return an {@code Integer} instance representing {@code i}.
* @since 1.5
*/
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
private static class LongCache {
private LongCache(){}
static final Long cache[] = new Long[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Long(i - 128);
}
}
/**
* Returns a {@code Long} instance representing the specified
* {@code long} value.
* If a new {@code Long} instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Long(long)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* Note that unlike the {@linkplain Integer#valueOf(int)
* corresponding method} in the {@code Integer} class, this method
* is <em>not</em> required to cache values within a particular
* range.
*
* @param l a long value.
* @return a {@code Long} instance representing {@code l}.
* @since 1.5
*/
public static Long valueOf(long l) {
final int offset = 128;
if (l >= -128 && l <= 127) { // will cache
return LongCache.cache[(int)l + offset];
}
return new Long(l);
}
上面的快取程式碼可以完美解釋下面的結果:
Integer a = 10;
Integer b = 10;
System.out.println(a == b);// 輸出 true
Float c = 10f;
Float d = 10f;
System.out.println(c == d);// 輸出 false
Double e = 1.2;
Double f = 1.2;
System.out.println(e == f);// 輸出 false
Integer a = 10;
Integer b = new Integer(10);
System.out.println(a==b);// false
//Integer a = 10;=>Integer a=Integer.valueOf(10);使用常量池中的物件,Integer b = new Integer(10);建立新物件
自動裝箱與拆箱
從位元組碼中,我們發現裝箱其實就是呼叫了 包裝類的valueOf()方法,拆箱其實就是呼叫了 xxxValue()方法。
- Integer i = 1 等價於 Integer i = Integer.valueOf(1)
- int n = i 等價於 int n = i.intValue();
- 如果頻繁拆裝箱的話,也會嚴重影響系統的效能。我們應該儘量避免不必要的拆裝箱操作。