Void & Boolean

基本类型类

Void

持有一个关键字void的伪类型 & 不可实例化

/**
 * The {@code Void} class is an uninstantiable placeholder class to hold a
 * reference to the {@code Class} object representing the Java keyword
 * void.
 *
 * 这个Void类是一个为了持有一个关键字void的Class引用的不可实例化的占位class
 *
 * @author  unascribed
 * @since   JDK1.1
 */
public final class Void {

    /**
     * The {@code Class} object representing the pseudo-type corresponding to
     * the keyword {@code void}.
     * Type是一个代表关键字void的伪类型
     */
    @SuppressWarnings("unchecked")
    public static final Class<Void> TYPE = (Class<Void>) Class.getPrimitiveClass("void");

    /*
     * The Void class cannot be instantiated.
     */
    private Void() {}
}

Boolean

采用享元模式，内置了两个变量Boolean,TRUE, Boolean.FALSE

注意：除非需要一个新的实例，否则基本不应该使用这个构造方法，
静态工厂方法（valueOf）通常是一个更好的选择, 看起来能大幅提升时空性能
valueOf 方法是享元模式的应用
享元模式参考博客https://www.cnblogs.com/adamjwh/p/9070107.html
简单介绍下
享元的核心是，共享对象，但是共享对象存在着线程安全问题，
所以想将需要共享的对象的属性拆分为可共享和不可共享两部分，
用不可共享部分作为key缓存共享对象。

Boolean的hashcode竟然是固定值1231是true，1237是false
提供了逻辑与、或、异或的方法

  /**
     * 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><sup>TM</sup></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.
     * 奇怪的方法，看了下调用，应该是用来读JVM属性并且判断是否开启某功能的
     * 用来返回系统属性是否为true，只有存在并且属性值为"true"（大小写不敏感），的时候返回true
     *
     * @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;
    }

/**
 * 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}.
 *
 * Boolean 类在对象中包装了一个基本类型boolean
 * Boolean 只包含一个类型是boolean的属性
 *
 * <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}.
 * 此外，这个类提供了很多对于处理boolean有用的方法，比如boolean和String互转
 *
 * @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. 从1.0.2开始使用serialVersionUID保证兼容性 */
    private static final long serialVersionUID = -3665804199014368530L;

    /**
     * Allocates a {@code Boolean} object representing the
     * {@code value} argument.
     * 分配一个代表value的Boolean对象
     *
     * <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>
     *
     * 注意：除非需要一个新的实例，否则基本不应该使用这个构造方法，
     * 静态工厂方法（valueOf）通常是一个更好的选择, 看起来能大幅提升时空性能
     * valueOf 方法是享元模式的应用
     * 享元模式参考博客https://www.cnblogs.com/adamjwh/p/9070107.html
     * 简单介绍下
     * 享元的核心是，共享对象，但是共享对象存在着线程安全问题，
     * 所以想将需要共享的对象的属性拆分为可共享和不可共享两部分，
     * 用不可共享部分作为key缓存共享对象。
     *
     * 另一个需要注意的点是，享元对象是个抽象类，可以有可以共享和不可以共享两个子类
     *
     *
     * @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}.
     *
     * 传入非空 & 忽略大小写为"true"时返回 Boolean.TRUE, 否则返回Boolean.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.
     *
     * 竟然有个静态方法toString
     * @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()}.
     *
     * 竟然是固定值1231是true，1237是false
     * @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><sup>TM</sup></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.
     * 奇怪的方法，看了下调用，应该是用来读JVM属性并且判断是否开启某功能的
     * 用来返回系统属性是否为true，只有存在并且属性值为"true"（大小写不敏感），的时候返回true
     *
     * @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
     *          说白了true=1 & false=0，return this.value - that.value
     * @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>
     * 静态对比方法
     * @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.
     * 逻辑与
     *
     * @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.
     * 逻辑或
     * @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.
     * 逻辑异或，代表两个值是不是不同，不同返回true，相同返回false
     * @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;
    }
}