Guava的不可變集合

1、不可變集合的優勢

不可變對象有不少優勢，包括：

• 當對象被不可信的庫調用時，不可變形式是安全的；
• 不可變對象被多個線程調用時，不存在競態條件問題
• 不可變集合不須要考慮變化，所以能夠節省時間和空間。全部不可變的集合都比它們的可變形式有更好的內存利用率（分析和測試細節）；
• 不可變對象由於有固定不變，能夠做爲常量來安全使用。

建立對象的不可變拷貝是一項很好的防護性編程技巧。Guava爲全部JDK標準集合類型和Guava新集合類型都提供了簡單易用的不可變版本。

摘自：http://ifeve.com/google-guava-immutablecollections/

2、建立不可變集合

1.使用of方法

如：I

mmutableList<String> ilist = ImmutableList.of("a","b","c","d","e");

2.使用copyOf

如：

List<String> slist = new ArrayList<String>(20);

slist.add("a");

slist.add("b");

slist.add("c");

slist.add("d");

slist.add("e");

//經過已有List建立不可變list

ImmutableList<String> ilist = ImmutableList.copyOf(slist);

3.使用Builder工具

如：

List<String> slist = new ArrayList<String>(20);

slist.add("a");

slist.add("b");

slist.add("c");

slist.add("d");

slist.add("e");

ImmutableList<String> ilist = ImmutableList.<String>builder().addAll(list).build();

3、如何實現不可變

1.對不可變集合調用addAll、add、set、remove都會拋出UnsupportedOperationException異常

以下是ImmutableList類的源碼：

/**

* Guaranteed to throw an exception and leave the list unmodified.

*

* @throws UnsupportedOperationException always

* @deprecated Unsupported operation.

*/

@CanIgnoreReturnValue

@Deprecated

@Override

public final boolean addAll(int index, Collection<? extends E> newElements) {

throw new UnsupportedOperationException();

}

 

/**

* Guaranteed to throw an exception and leave the list unmodified.

*

* @throws UnsupportedOperationException always

* @deprecated Unsupported operation.

*/

@CanIgnoreReturnValue

@Deprecated

@Override

public final E set(int index, E element) {

throw new UnsupportedOperationException();

}

 

/**

* Guaranteed to throw an exception and leave the list unmodified.

*

* @throws UnsupportedOperationException always

* @deprecated Unsupported operation.

*/

@Deprecated

@Override

public final void add(int index, E element) {

throw new UnsupportedOperationException();

}

 

/**

* Guaranteed to throw an exception and leave the list unmodified.

*

* @throws UnsupportedOperationException always

* @deprecated Unsupported operation.

*/

@CanIgnoreReturnValue

@Deprecated

@Override

public final E remove(int index) {

throw new UnsupportedOperationException();

}

 

2.集合中存儲的可變對象仍可改變

Users[] arr = new Users[3];

arr[0] = new Users();

arr[0].setId(1);//id=1

arr[1] = new Users();

arr[1].setId(2);

arr[2] = new Users();

arr[2].setId(3);

//copyof 是淺拷貝，所以集合中存放可變對象時，原對象的屬性修改了，集合中對象的屬性也會修改

ImmutableList<Users> alist = ImmutableList.copyOf(arr);

arr[0].setId(5);

System.out.println(alist.get(0).getId());//id=5

 

4、源碼中有趣的地方

不可變集合能夠經過of方法來建立，of方法是包含12個重載方法以下：

@SuppressWarnings("unchecked")

public static <E> ImmutableList<E> of() {

return (ImmutableList<E>) EMPTY;

}

 

public static <E> ImmutableList<E> of(E element) {

return new SingletonImmutableList<E>(element);

}

 

public static <E> ImmutableList<E> of(E e1, E e2) {

return construct(e1, e2);

}

 

public static <E> ImmutableList<E> of(E e1, E e2, E e3) {

return construct(e1, e2, e3);

}

 

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4) {

return construct(e1, e2, e3, e4);

}

 

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5) {

return construct(e1, e2, e3, e4, e5);

}

 

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {

return construct(e1, e2, e3, e4, e5, e6);

}

 

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {

return construct(e1, e2, e3, e4, e5, e6, e7);

}

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {

return construct(e1, e2, e3, e4, e5, e6, e7, e8);

}

public static <E> ImmutableList<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {

return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9);

}

public static <E> ImmutableList<E> of(

E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10) {

return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);

}

 

public static <E> ImmutableList<E> of(

E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11) {

return construct(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10, e11);

}

 

// These go up to eleven. After that, you just get the varargs form, and

// whatever warnings might come along with it. :(

 

@SafeVarargs // For Eclipse. For internal javac we have disabled this pointless type of warning.

public static <E> ImmutableList<E> of(

E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10, E e11, E e12, E... others) {

Object[] array = new Object[12 + others.length];

array[0] = e1;

array[1] = e2;

array[2] = e3;

array[3] = e4;

array[4] = e5;

array[5] = e6;

array[6] = e7;

array[7] = e8;

array[8] = e9;

array[9] = e10;

array[10] = e11;

array[11] = e12;

System.arraycopy(others, 0, array, 12, others.length);

return construct(array);

}

 

java方法是支持可變參數的，徹底能夠寫成，public static <E> ImmutableList<E> of(E... others);這樣實現的緣由大神在註釋中給出了，爲了不調用者處理類型檢測警告。

當方法聲明爲泛型可變參數時，調用該方法時，編譯器就會提示該警告，在JDK1.6以前，能夠在方法調用的地方添加@SuppressWarnings("unchecked")註解，但這樣比較麻煩，項目組每一個調用的地方都須要處理，固然也能夠忽略這個問題。

此處ImmutableList中of方法的12個重載的前11個就是爲了儘可能減小類型檢測警告，大神仍是很貼心滴爲咱們寫了11個重載，而第12個方法經過JDK1.7的新註解@SafeVarargs 來避免類型檢測警告。

@SafeVarargs的用法請參考：http://book.51cto.com/art/201205/339154.htm