JDK源码分析-java.lang.Boolean

闲来无事，开始研究JDK 1.5源码，先找了一个最简单的java.lang.Boolean开始解剖。

首先我们剔除所有的方法和静态变量，Boolean的核心代码如下：

public final class Boolean implements java.io.Serializable,Comparable {
private final boolean value;
}

很明显，凡是成员变量都是final类型的，一定是immutable class，这个Boolean和String一样，一旦构造函数执行完毕，实例的状态就不能再改变了。

Boolean的构造方法有两个：

public Boolean(boolean value) {
this.value = value;
}

public Boolean(String s) {
this(toBoolean(s));
}

另外注意到Boolean类实际上只有两种不同状态的实例：一个包装true，一个包装false，Boolean又是immutable class，所以在内存中相同状态的Boolean实例完全可以共享，不必用new创建很多实例。因此Boolean class还提供两个静态变量：

public static final Boolean TRUE = new Boolean(true);
public static final Boolean FALSE = new Boolean(false);

这两个变量在Class Loader装载时就被实例化，并且申明为final，不能再指向其他实例。

提供这两个静态变量是为了让开发者直接使用这两个变量而不是每次都new一个Boolean，这样既节省内存又避免了创建一个新实例的时间开销。

因此，用

Boolean b = Boolean.TRUE;

比

Boolean b = new Boolean(true);

要好得多。

如果遇到下面的情况：

Boolean b = new Boolean(var);

一定要根据一个boolean变量来创建Boolean实例怎么办？推荐使用Boolean提供的静态工厂方法：

Boolean b = Boolean.valueOf(var);

这样就可以避免创建新的实例。看看valueOf()静态方法：

public static Boolean valueOf(boolean b) {
return (b ? TRUE : FALSE);
}

这个静态工厂方法返回的仍然是两个静态变量TRUE和FALSE之一，而不是new一个Boolean出来。虽然Boolean非常简单，占用的内存也很少，但是一个复杂的类用new创建实例的开销可能非常大，而且，使用工厂方法可以方便的实现缓存实例，这对客户端是透明的。所以，能用工厂方法就不要使用new。

和Boolean只有两种状态不同，Integer也是immutable class，但是状态上亿种，不可能用静态实例缓存所有状态。不过，SUN的工程师还是作了一点优化，Integer类缓存了-128到127这256个状态的Integer，如果使用Integer.valueOf(int i)，传入的int范围正好在此内，就返回静态实例。

hashCode()方法很奇怪，两种Boolean的hash code分别是1231和1237。估计写Boolean.java的人对这两个数字有特别偏好：

public int hashCode() {
return value ? 1231 : 1237;
}

equals()方法也很简单，只有Boolean类型的Object并且value相等才返true：

public boolean equals(Object obj) {
if (obj instanceof Boolean) {
return value == ((Boolean)obj).booleanValue();
}
return false;
}

很多人写equals()总是在第一行写一个null判断：

if (obj==null)
return false;

其实完全没有必要，因为如果obj==null，下一行的if (obj instanceof Type)就肯定返回false，因为(null instanceof AnyType)永远是false。

详细内容请参考《Effective Java》第7条：Obey the general contract when overriding equals。

总结

如果一个类只有有限的几种状态，考虑用几个final的静态变量来表示不同状态的实例。例如编写一个Weekday类，状态只有7个，就不要让用户写new Weekday(1)，直接提供Weekday.MONDAY即可。

要防止用户使用new生成实例，就取消public构造方法，用户要获得静态实例的引用有两个方法：如果申明了public static var，就可以直接访问，比如Boolean.TRUE，第二个方法是通过静态工厂方法：Boolean.valueOf(?)。

如果不提供public构造方法，让用户只能通过上面的方法获得静态变量的引用，还可以大大简化equals()方法：

public boolean equals(Object obj) {
return this==obj;
}

http://www.liaoxuefeng.com/article/0013738760210200c0b2b12da1c470a8d771dc7f218e89d000
源代码：

/*
* Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/

package java.lang;

/**
* The Boolean class wraps a value of the primitive type
* {@code boolean} in an object. An object of type
* {@code Boolean} contains a single field whose type is
* {@code boolean}.
* <p>
* In addition, this class provides many methods for
* converting a {@code boolean} to a {@code String} and a
* {@code String} to a {@code boolean}, as well as other
* constants and methods useful when dealing with a
* {@code boolean}.
*
* @author Arthur van Hoff
* @since JDK1.0
*/
public final class Boolean implements java.io.Serializable,
Comparable<Boolean>
{
/**
* The {@code Boolean} object corresponding to the primitive
* value {@code true}.
*/
public static final Boolean TRUE = new Boolean(true);

/**
* The {@code Boolean} object corresponding to the primitive
* value {@code false}.
*/
public static final Boolean FALSE = new Boolean(false);

/**
* The Class object representing the primitive type boolean.
*
* @since JDK1.1
*/
@SuppressWarnings("unchecked")
public static final Class<Boolean> TYPE = (Class<Boolean>) Class.getPrimitiveClass("boolean");

/**
* The value of the Boolean.
*
* @serial
*/
private final boolean value;

/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -3665804199014368530L;

/**
* Allocates a {@code Boolean} object representing the
* {@code value} argument.
*
* <p><b>Note: It is rarely appropriate to use this constructor.
* Unless a <i>new</i> instance is required, the static factory
* {@link #valueOf(boolean)} is generally a better choice. It is
* likely to yield significantly better space and time performance.</b>
*
* @param value the value of the {@code Boolean}.
*/
public Boolean(boolean value) {
this.value = value;
}

/**
* Allocates a {@code Boolean} object representing the value
* {@code true} if the string argument is not {@code null}
* and is equal, ignoring case, to the string {@code "true"}.
* Otherwise, allocate a {@code Boolean} object representing the
* value {@code false}. Examples:<p>
* {@code new Boolean("True")} produces a {@code Boolean} object
* that represents {@code true}.<br>
* {@code new Boolean("yes")} produces a {@code Boolean} object
* that represents {@code false}.
*
* @param s the string to be converted to a {@code Boolean}.
*/
public Boolean(String s) {
this(parseBoolean(s));
}

/**
* Parses the string argument as a boolean. The {@code boolean}
* returned represents the value {@code true} if the string argument
* is not {@code null} and is equal, ignoring case, to the string
* {@code "true"}. <p>
* Example: {@code Boolean.parseBoolean("True")} returns {@code true}.<br>
* Example: {@code Boolean.parseBoolean("yes")} returns {@code false}.
*
* @param s the {@code String} containing the boolean
* representation to be parsed
* @return the boolean represented by the string argument
* @since 1.5
*/
public static boolean parseBoolean(String s) {
return ((s != null) && s.equalsIgnoreCase("true"));
}

/**
* Returns the value of this {@code Boolean} object as a boolean
* primitive.
*
* @return the primitive {@code boolean} value of this object.
*/
public boolean booleanValue() {
return value;
}

/**
* Returns a {@code Boolean} instance representing the specified
* {@code boolean} value. If the specified {@code boolean} value
* is {@code true}, this method returns {@code Boolean.TRUE};
* if it is {@code false}, this method returns {@code Boolean.FALSE}.
* If a new {@code Boolean} instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Boolean(boolean)}, as this method is likely to yield
* significantly better space and time performance.
*
* @param b a boolean value.
* @return a {@code Boolean} instance representing {@code b}.
* @since 1.4
*/
public static Boolean valueOf(boolean b) { return (b ? TRUE : FALSE); }
/**
* Returns a {@code Boolean} with a value represented by the
* specified string. The {@code Boolean} returned represents a
* true value if the string argument is not {@code null}
* and is equal, ignoring case, to the string {@code "true"}.
*
* @param s a string.
* @return the {@code Boolean} value represented by the string.
*/
public static Boolean valueOf(String s) {
return parseBoolean(s) ? TRUE : FALSE;
}

/**
* Returns a {@code String} object representing the specified
* boolean. If the specified boolean is {@code true}, then
* the string {@code "true"} will be returned, otherwise the
* string {@code "false"} will be returned.
*
* @param b the boolean to be converted
* @return the string representation of the specified {@code boolean}
* @since 1.4
*/
public static String toString(boolean b) {
return b ? "true" : "false";
}

/**
* Returns a {@code String} object representing this Boolean's
* value. If this object represents the value {@code true},
* a string equal to {@code "true"} is returned. Otherwise, a
* string equal to {@code "false"} is returned.
*
* @return a string representation of this object.
*/
public String toString() {
return value ? "true" : "false";
}

/**
* Returns a hash code for this {@code Boolean} object.
*
* @return the integer {@code 1231} if this object represents
* {@code true}; returns the integer {@code 1237} if this
* object represents {@code false}.
*/
@Override
public int hashCode() {
return Boolean.hashCode(value);
}

/**
* Returns a hash code for a {@code boolean} value; compatible with
* {@code Boolean.hashCode()}.
*
* @param value the value to hash
* @return a hash code value for a {@code boolean} value.
* @since 1.8
*/
public static int hashCode(boolean value) {
return value ? 1231 : 1237;
}

/**
* Returns {@code true} if and only if the argument is not
* {@code null} and is a {@code Boolean} object that
* represents the same {@code boolean} value as this object.
*
* @param obj the object to compare with.
* @return {@code true} if the Boolean objects represent the
* same value; {@code false} otherwise.
*/
public boolean equals(Object obj) { if (obj instanceof Boolean) { return value == ((Boolean)obj).booleanValue(); } return false; }
/**
* Returns {@code true} if and only if the system property
* named by the argument exists and is equal to the string
* {@code "true"}. (Beginning with version 1.0.2 of the
* Java<small>^TM</small> platform, the test of
* this string is case insensitive.) A system property is accessible
* through {@code getProperty}, a method defined by the
* {@code System} class.
* <p>
* If there is no property with the specified name, or if the specified
* name is empty or null, then {@code false} is returned.
*
* @param name the system property name.
* @return the {@code boolean} value of the system property.
* @throws SecurityException for the same reasons as
* {@link System#getProperty(String) System.getProperty}
* @see java.lang.System#getProperty(java.lang.String)
* @see java.lang.System#getProperty(java.lang.String, java.lang.String)
*/
public static boolean getBoolean(String name) {
boolean result = false;
try {
result = parseBoolean(System.getProperty(name));
} catch (IllegalArgumentException | NullPointerException e) {
}
return result;
}

/**
* Compares this {@code Boolean} instance with another.
*
* @param b the {@code Boolean} instance to be compared
* @return zero if this object represents the same boolean value as the
* argument; a positive value if this object represents true
* and the argument represents false; and a negative value if
* this object represents false and the argument represents true
* @throws NullPointerException if the argument is {@code null}
* @see Comparable
* @since 1.5
*/
public int compareTo(Boolean b) {
return compare(this.value, b.value);
}

/**
* Compares two {@code boolean} values.
* The value returned is identical to what would be returned by:
* <pre>
* Boolean.valueOf(x).compareTo(Boolean.valueOf(y))
* </pre>
*
* @param x the first {@code boolean} to compare
* @param y the second {@code boolean} to compare
* @return the value {@code 0} if {@code x == y};
* a value less than {@code 0} if {@code !x && y}; and
* a value greater than {@code 0} if {@code x && !y}
* @since 1.7
*/
public static int compare(boolean x, boolean y) {
return (x == y) ? 0 : (x ? 1 : -1);
}

/**
* Returns the result of applying the logical AND operator to the
* specified {@code boolean} operands.
*
* @param a the first operand
* @param b the second operand
* @return the logical AND of {@code a} and {@code b}
* @see java.util.function.BinaryOperator
* @since 1.8
*/
public static boolean logicalAnd(boolean a, boolean b) {
return a && b;
}

/**
* Returns the result of applying the logical OR operator to the
* specified {@code boolean} operands.
*
* @param a the first operand
* @param b the second operand
* @return the logical OR of {@code a} and {@code b}
* @see java.util.function.BinaryOperator
* @since 1.8
*/
public static boolean logicalOr(boolean a, boolean b) {
return a || b;
}

/**
* Returns the result of applying the logical XOR operator to the
* specified {@code boolean} operands.
*
* @param a the first operand
* @param b the second operand
* @return the logical XOR of {@code a} and {@code b}
* @see java.util.function.BinaryOperator
* @since 1.8
*/
public static boolean logicalXor(boolean a, boolean b) {
return a ^ b;
}
}