private static long getCounterResult() {
    Long sum = 0L;
    final int length = Integer.MAX_VALUE;
    for (int i = 0; i < length; i++) {
        sum += i;
    }

    return sum;
}

public static void main(String[] args) {
    long startCountTime = System.currentTimeMillis();
    long result = getCounterResult();
    long endCountTime = System.currentTimeMillis();
    System.out.println("result = " + result + ", and take up time : " + (endCountTime - startCountTime) / 1000 + "s");
}

int a = 3;
Integer b = a;

Integer b = new Integer(7);
int a = b;

/**
 * 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) {
	  // 如果i的值大于-128小于127则返回一个缓冲区中的一个Integer对象
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
        
    // 否则返回 new 一个Integer 对象
    return new Integer(i);
}

Integer i = 100;

Integer i = Integer.valueOf(100);

public static void main(String[] args) {
	// ⓵
	Integer a = new Integer(121);
	Integer b = new Integer(121);
	System.out.println(a == b);
	
	// ⓶
	Integer c = 121;
	Integer d = 121;
	System.out.println(c == d);
	
	// ⓷
	Integer e = 129;
	Integer f = 129;
	System.out.println(e == f);
	
	// ⓸
	int g = 50;
	Integer h = new Integer(50);
	System.out.println(g == h);
}

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);
}

Long sum = 0L;

Long sum = Long.valueof(0);

private static long getCounterResult() {
    // 修改为普通的基本类型数据
    long sum = 0L;
    final int length = Integer.MAX_VALUE;
    for (int i = 0; i < length; i++) {
        sum += i;
    }

    return sum;
}

public static void main(String[] args) {
    long startCountTime = System.currentTimeMillis();
    long result = getCounterResult();
    long endCountTime = System.currentTimeMillis();
    System.out.println("result = " + result + ", and take up time : " + (endCountTime - startCountTime) / 1000 + "s");
}

public enum Color {
	RED, GREEN, BLUE, YELLOW;
}

public enum Color {
	RED(), GREEN(), BLUE(), YELLOW();
}

public class Color {
	private static final Color RED = new Color();
	private static final Color GREEN = new Color();
	private static final Color BLUE = new Color();
	private static final Color YELLOW = new Color();
}

public enum Color {
	RED("red color", 0), GREEN("green color", 1), 
	BLUE("blue color", 2), YELLOW("yellow color", 3);
	
	Color(String name, int id) {
		_name = name;
		_id = id;
	}
	
	String _name;
	int _id;
}

public enum Color {
		RED("red color", 0), GREEN("green color", 1), 
		BLUE("blue color", 2), YELLOW("yellow color", 3);
}

public enum Color {
	RED("red color", 0), GREEN("green color", 1),
	BLUE("blue color", 2), YELLOW("yellow color", 3);
	
	Color(String name, int id) {
		_name = name;
		_id = id;
	}
	
	private String _name;
	private int _id;
		
	public void setName(String name) {
		_name = name;
	}
		
	public void setId(int id) {
		_id = id;
	}
	
	public String getName() {
		return _name;
	}
	
	public int getId() {
		return _id;
	}
}

public enum Color {
	RED("red color", 0), GREEN("green color", 1),
	BLUE("blue color", 2), YELLOW("yellow color", 3);
	
	Color(String name, int id) {
		_name = name;
		_id = id;
	}
	
	private String _name;
	private int _id;
		
	public String getName() {
		return _name;
	}
	
	public int getId() {
		return _id;
	}
}

public static enum Color {
	RED("red color", 0), GREEN("green color", 1),
	BLUE("blue color", 2), YELLOW("yellow color", 3);
	
	private String _name;
	private int _id;
	
	private Color(String name, int id) {
		this._name = name;
		this._id = id;
	}
	
	public String getName() {
		return this._name;
	}
	
	public int getId() {
		return this._id;
	}
}

package mark.demo;

import java.util.ArrayList;
import java.util.List;

public class Color {
	private static final Color RED = new Color("red color", 0);
	private static final Color GREEN = new Color("green color", 1);
	private static final Color BLUE = new Color("blue color", 2);
	private static final Color YELLOW = new Color("yellow color", 3);

	private final String _name;
	private final int _id;

	private Color(String name, int id) {
		_name = name;
		_id = id;
	}

	public String getName() {
		return _name;
	}

	public int getId() {
		return _id;
	}

	public static List<Color> values() {
		List<Color> list = new ArrayList<Color>();
		list.add(RED);
		list.add(GREEN);
		list.add(BLUE);
		list.add(YELLOW);
		return list;
	}

	@Override
	public String toString() {
		return "the color _name=" + _name + ", _id=" + _id;
	}
}

package java.lang;

import java.io.Serializable;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;

/**
 * This is the common base class of all Java language enumeration types.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @version %I%, %G%
 * @since   1.5
 */
public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
    /**
     * The name of this enum constant, as declared in the enum declaration.
     * Most programmers should use the {@link #toString} method rather than
     * accessing this field.
     */
    private final String name;

    /**
     * Returns the name of this enum constant, exactly as declared in its
     * enum declaration.
     * 
     * <b>Most programmers should use the {@link #toString} method in
     * preference to this one, as the toString method may return
     * a more user-friendly name.</b>  This method is designed primarily for
     * use in specialized situations where correctness depends on getting the
     * exact name, which will not vary from release to release.
     *
     * @return the name of this enum constant
     */
    public final String name() {
		return name;
    }

    /**
     * The ordinal of this enumeration constant (its position
     * in the enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     * 
     * Most programmers will have no use for this field.  It is designed
     * for use by sophisticated enum-based data structures, such as
     * {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     */
    private final int ordinal;

    /**
     * Returns the ordinal of this enumeration constant (its position
     * in its enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     * 
     * Most programmers will have no use for this method.  It is
     * designed for use by sophisticated enum-based data structures, such
     * as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     *
     * @return the ordinal of this enumeration constant
     */
    public final int ordinal() {
	return ordinal;
    }

    /**
     * Sole constructor.  Programmers cannot invoke this constructor.
     * It is for use by code emitted by the compiler in response to
     * enum type declarations.
     *
     * @param name - The name of this enum constant, which is the identifier
     *               used to declare it.
     * @param ordinal - The ordinal of this enumeration constant (its position
     *         in the enum declaration, where the initial constant is assigned
     *         an ordinal of zero).
     */
    protected Enum(String name, int ordinal) {
		this.name = name;
		this.ordinal = ordinal;
    }

    /**
     * Returns the name of this enum constant, as contained in the
     * declaration.  This method may be overridden, though it typically
     * isn't necessary or desirable.  An enum type should override this
     * method when a more "programmer-friendly" string form exists.
     *
     * @return the name of this enum constant
     */
    public String toString() {
		return name;
    }

    /**
     * Returns true if the specified object is equal to this
     * enum constant.
     *
     * @param other the object to be compared for equality with this object.
     * @return  true if the specified object is equal to this
     *          enum constant.
     */
    public final boolean equals(Object other) { 
        return this==other;
    }

    /**
     * Returns a hash code for this enum constant.
     *
     * @return a hash code for this enum constant.
     */
    public final int hashCode() {
        return super.hashCode();
    }

    /**
     * Throws CloneNotSupportedException.  This guarantees that enums
     * are never cloned, which is necessary to preserve their "singleton"
     * status.
     *
     * @return (never returns)
     */
    protected final Object clone() throws CloneNotSupportedException {
		throw new CloneNotSupportedException();
    }

    /**
     * Compares this enum with the specified object for order.  Returns a
     * negative integer, zero, or a positive integer as this object is less
     * than, equal to, or greater than the specified object.
     * 
     * Enum constants are only comparable to other enum constants of the
     * same enum type.  The natural order implemented by this
     * method is the order in which the constants are declared.
     */
    public final int compareTo(E o) {
	Enum other = (Enum)o;
	Enum self = this;
	if (self.getClass() != other.getClass() && // optimization
      self.getDeclaringClass() != other.getDeclaringClass())
	    throw new ClassCastException();
		return self.ordinal - other.ordinal;
    }

    /**
     * Returns the Class object corresponding to this enum constant's
     * enum type.  Two enum constants e1 and  e2 are of the
     * same enum type if and only if
     *   e1.getDeclaringClass() == e2.getDeclaringClass().
     * (The value returned by this method may differ from the one returned
     * by the {@link Object#getClass} method for enum constants with
     * constant-specific class bodies.)
     *
     * @return the Class object corresponding to this enum constant's
     *     enum type
     */
    public final Class<E> getDeclaringClass() {
		Class clazz = getClass();
		Class zuper = clazz.getSuperclass();
		return (zuper == Enum.class) ? clazz : zuper;
    }

    /**
     * Returns the enum constant of the specified enum type with the
     * specified name.  The name must match exactly an identifier used
     * to declare an enum constant in this type.  (Extraneous whitespace
     * characters are not permitted.) 
     *
     * @param enumType the <tt>Class</tt> object of the enum type from which
     *      to return a constant
     * @param name the name of the constant to return
     * @return the enum constant of the specified enum type with the
     *      specified name
     * @throws IllegalArgumentException if the specified enum type has
     *         no constant with the specified name, or the specified
     *         class object does not represent an enum type
     * @throws NullPointerException if <tt>enumType</tt> or <tt>name</tt>
     *         is null
     * @since 1.5
     */
    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum const " + enumType +"." + name);
    }

    /**
      * prevent default deserialization
      */
    private void readObject(ObjectInputStream in) throws IOException,
        ClassNotFoundException {
            throw new InvalidObjectException("can't deserialize enum");
    }

    private void readObjectNoData() throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    /**
     * enum classes cannot have finalize methods.
     */
    protected final void finalize() { }
}