1、一个Interface的方所有法访问权限（visibility)自动被声明为public,确却的说，一个Interface的所有方法只能是public的，你可以显式声明一个方法是public（不推荐),但是不能声明它是private或protected.但是当一个类实现某个接口，定义接口的方法时，必须且只能声明为public，否则编译将通不过。
2、接口不能实现方法（implement method),只能声明。接口可以只定义常量但不声明任何方法。
3、Interface不能有实例域（instance fields)或静态方法（static method),但可以定义常量(define constants),常量自动设为public static final，可以通过类命直接引用常量，例如
ImplementClass.z
可以通过接口命和常量名直接访问常量：
FirstInterface.z
4、一个
非抽象类
（注意是非抽象类！）实现一个接口时，必须实现接口的所有方法，抽象类则不必实现所有方法。
5、不能使用new操作符实例化一个接口，但可以声明一个接口变量，该变量必须引用（refer to)一个实现该接口的类的对象。可以使用 instanceof 检查一个对象是否实现了某个特定的接口。例如：
if(anObject instanceof Comparable){
} 6、接口可以被另一个接口继承（但是final好像不能修饰interface，编译通不过，以后慢慢研究~~)
7、标记接口（tagging interface, marker interface)没有方法，使用它的唯一目的是可以用instanceof 进行类型检查（Horstmann说了，不鼓励用这种技术,^_^）
8、
方法的名字和参数列表被称为方法的签名（signature)，实现一个接口以为着要用完全相同的签名实现每个方法。因此实现接口方法时，一定要保证返回类型的兼容性。允许实现类的实现方法返回类型定义为原返回类型的子类型。这个跟继承中子类覆盖父类方法很相似。
8、例子：
FirstInterface.java
interface FirstInterface
{
int x = 20; //int x; 是不允许的
public int y = 21; //private int y=21; 或protected int y=22;均为非法声明
static int z = 22;
public static int u = 23;
void foobar();
}SecondInterface.java
1 /**
2 * 继承了FirstInterface的所有常量和方法
3 */
4 interface SecondInterface extends FirstInterface
5 {
6 int squad(int x);
7 } ImplementClass.java
1 /**
2 * 类ImplementClass 必须实现FirstInterface和
3 * SecondInterface的所有方法
4 */
5 class ImplementClass implements SecondInterface
6 {
7 public void foobar()
8 {
9 System.out.println("I love you!");
10 }
11
12 public int squad(int x)
13 {
14 return x*x;
15 }
16 } Main.java
1 public class Main
2 {
3 public static void main(String [] args)
4 {
5 FirstInterface ic;
6 ic = new ImplementClass();
7 SecondInterface sic = new ImplementClass();
8 System.out.println(ic.x);
9 System.out.println(sic.y);
10 System.out.println(ImplementClass.z);
11 ic.foobar();
12 sic.foobar();
13 }
14 } 运行结果：
20
21
22
I love you!
I love you!
9、匿名内部类(anonymous inner class):
注意一个特别的例子
AnonymousInnerClass.java
import java.util.Comparator;
public class AnonymousInnerClass
{
public static void main(String [] args)
{
Comparator<String> sizeOrder = new Comparator<String>()
{
public int compare(String s1, String s2)
{
return s1.length() < s2.length() ? -1 : s1.length() > s2.length() ? 1 : s1.compareTo(s2);
}
};
System.out.println(sizeOrder.compare("Jafe", "Lee"));
}
} 而java.util.Comparator的定义为
Comparator.java
/*
* @(#)Comparator.java 1.26 06/04/21
*
* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
/**
* A comparison function, which imposes a <i>total ordering</i> on some
* collection of objects. Comparators can be passed to a sort method (such
* as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
* Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
* over the sort order. Comparators can also be used to control the order of
* certain data structures (such as {@link SortedSet sorted sets} or {@link
* SortedMap sorted maps}), or to provide an ordering for collections of
* objects that don't have a {@link Comparable natural ordering}.<p>
*
* The ordering imposed by a comparator <tt>c</tt> on a set of elements
* <tt>S</tt> is said to be <i>consistent with equals</i> if and only if
* <tt>c.compare(e1, e2)==0</tt> has the same boolean value as
* <tt>e1.equals(e2)</tt> for every <tt>e1</tt> and <tt>e2</tt> in
* <tt>S</tt>.<p>
*
* Caution should be exercised when using a comparator capable of imposing an
* ordering inconsistent with equals to order a sorted set (or sorted map).
* Suppose a sorted set (or sorted map) with an explicit comparator <tt>c</tt>
* is used with elements (or keys) drawn from a set <tt>S</tt>. If the
* ordering imposed by <tt>c</tt> on <tt>S</tt> is inconsistent with equals,
* the sorted set (or sorted map) will behave "strangely." In particular the
* sorted set (or sorted map) will violate the general contract for set (or
* map), which is defined in terms of <tt>equals</tt>.<p>
*
* For example, suppose one adds two elements {@code a} and {@code b} such that
* {@code (a.equals(b) && c.compare(a, b) != 0)}
* to an empty {@code TreeSet} with comparator {@code c}.
* The second {@code add} operation will return
* true (and the size of the tree set will increase) because {@code a} and
* {@code b} are not equivalent from the tree set's perspective, even though
* this is contrary to the specification of the
* {@link Set#add Set.add} method.<p>
*
* Note: It is generally a good idea for comparators to also implement
* <tt>java.io.Serializable</tt>, as they may be used as ordering methods in
* serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
* order for the data structure to serialize successfully, the comparator (if
* provided) must implement <tt>Serializable</tt>.<p>
*
* For the mathematically inclined, the <i>relation</i> that defines the
* <i>imposed ordering</i> that a given comparator <tt>c</tt> imposes on a
* given set of objects <tt>S</tt> is:<pre>
* {(x, y) such that c.compare(x, y) <= 0}.
* </pre> The <i>quotient</i> for this total order is:<pre>
* {(x, y) such that c.compare(x, y) == 0}.
* </pre>
*
* It follows immediately from the contract for <tt>compare</tt> that the
* quotient is an <i>equivalence relation</i> on <tt>S</tt>, and that the
* imposed ordering is a <i>total order</i> on <tt>S</tt>. When we say that
* the ordering imposed by <tt>c</tt> on <tt>S</tt> is <i>consistent with
* equals</i>, we mean that the quotient for the ordering is the equivalence
* relation defined by the objects' {@link Object#equals(Object)
* equals(Object)} method(s):<pre>
* {(x, y) such that x.equals(y)}. </pre><p>
*
* This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <T> the type of objects that may be compared by this comparator
*
* @author Josh Bloch
* @author Neal Gafter
* @version 1.26, 04/21/06
* @see Comparable
* @see java.io.Serializable
* @since 1.2
*/
public interface Comparator<T> {
/**
* Compares its two arguments for order. Returns a negative integer,
* zero, or a positive integer as the first argument is less than, equal
* to, or greater than the second.<p>
*
* In the foregoing description, the notation
* <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
* <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
* <tt>0</tt>, or <tt>1</tt> according to whether the value of
* <i>expression</i> is negative, zero or positive.<p>
*
* The implementor must ensure that <tt>sgn(compare(x, y)) ==
* -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This
* implies that <tt>compare(x, y)</tt> must throw an exception if and only
* if <tt>compare(y, x)</tt> throws an exception.)<p>
*
* The implementor must also ensure that the relation is transitive:
* <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies
* <tt>compare(x, z)>0</tt>.<p>
*
* Finally, the implementor must ensure that <tt>compare(x, y)==0</tt>
* implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all
* <tt>z</tt>.<p>
*
* It is generally the case, but <i>not</i> strictly required that
* <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking,
* any comparator that violates this condition should clearly indicate
* this fact. The recommended language is "Note: this comparator
* imposes orderings that are inconsistent with equals."
*
* @param o1 the first object to be compared.
* @param o2 the second object to be compared.
* @return a negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater than the
* second.
* @throws ClassCastException if the arguments' types prevent them from
* being compared by this comparator.
*/
int compare(T o1, T o2);
/**
*
* Indicates whether some other object is "equal to" this
* comparator. This method must obey the general contract of
* {@link Object#equals(Object)}. Additionally, this method can return
* <tt>true</tt> <i>only</i> if the specified object is also a comparator
* and it imposes the same ordering as this comparator. Thus,
* <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1,
* o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference
* <tt>o1</tt> and <tt>o2</tt>.<p>
*
* Note that it is <i>always</i> safe <i>not</i> to override
* <tt>Object.equals(Object)</tt>. However, overriding this method may,
* in some cases, improve performance by allowing programs to determine
* that two distinct comparators impose the same order.
*
* @param obj the reference object with which to compare.
* @return <code>true</code> only if the specified object is also
* a comparator and it imposes the same ordering as this
* comparator.
* @see Object#equals(Object)
* @see Object#hashCode()
*/
boolean equals(Object obj);
}
我敢开始看觉得奇怪，好像在匿名内部类中没有实现方法boolean equals(Object obj);后来仔细一想，其实所有的类都是Object的子类，而Object正好有该方法的实现，所以，即使没有实现该方法也是合法的。