Difference

Arrays

Lists

1

Covariant

Invariant

2

Reified at runtime

Erased at run time

3

Runtime type safety

Compile time type safety

non-reifiable types

E, List<E> and List<String>

whose runtime representation contains less information than its compile-time representation.

Reifiable types

List<?> and Map<?,?>

Reverse to above.

This code fragment is legal:

// Fails at runtime!

Object[] objectArray = new Long[1];

objectArray[0] = "I don't fit in"; // Throws ArrayStoreException

but this one is not:

// Won't compile!

List<Object> ol = new ArrayList<Long>(); // Incompatible types

ol.add("I don't fit in");

Prohibition on generic array creation

  1. Not possible for a generic type to return an array of its element type.
  2. Not possible for using varargs methods in combination with generic types.( This is because every time you invoke a varargs method, an array is created to hold the varargs parameters. Use warning suppress to deal with it if it's required.).

None of these array creation expressions are legal:

new List<E>[],

new List<String>[] ,

new E[].

All will result in generic array creation errors at compile time.

// Why generic array creation is illegal - won't compile!

List<String>[] stringLists = new List<String>[1]; // (1)

List<Integer> intList = Arrays.asList(42); // (2)

Object[] objects = stringLists; // (3)

objects[0] = intList; // (4)

String s = stringLists[0].get(0); // (5)

Let's pretend that line 1, which creates a generic array, is legal. Line 2 creates and initializes a List<Integer>containing a single element. Line 3 stores the List<String>array into an Object array variable, which is legal because arrays are covariant. Line 4 stores the List<Integer>into the sole element of the Object array, which succeeds because generics are implemented by erasure: the runtime type of a List<Integer>instance is simply List, and the runtime type of aList<String>[]instance is List[], so this assignment doesn't generate an ArrayStoreException. Now we're in trouble. We've stored a List<Integer> instance into an array that is declared to hold only List<String>instances. In line 5, we retrieve the sole element from the sole list in this array. The compiler automatically casts the retrieved element to String, but it's an Integer, so we get a ClassCastExceptionat runtime. In order to prevent this from happening, line 1 (which creates a generic array) generates a compile-time error.

Use the Lists instead of the array to check the type safety at compile time

// List-based generic reduction

static <E> E reduce(List<E> list, Function<E> f, E initVal) {

List<E> snapshot;

synchronized(list) {

snapshot = new ArrayList<E>(list);

}

E result = initVal;

for (E e : snapshot)

result = f.apply(result, e);

return result;

}

Summary

If you find yourself mixing them and getting compile-time errors or warnings, your first impulse should be to replace the arrays with lists.

Effective Java 25 Prefer lists to arrays的更多相关文章

  1. Effective Java 53 Prefer interfaces to reflection

    Disadvantage of reflection You lose all the benefits of compile-time type checking, including except ...

  2. Effective Java 69 Prefer concurrency utilities to wait and notify

    Principle Use the higher-level concurrency utilities instead of wait and notify for easiness. Use Co ...

  3. Effective Java 35 Prefer annotations to naming patterns

    Disadvantages of naming patterns Typographical errors may result in silent failures. There is no way ...

  4. Effective Java 46 Prefer for-each loops to traditional for loops

    Prior to release 1.5, this was the preferred idiom for iterating over a collection: // No longer the ...

  5. Effective Java 68 Prefer executors and tasks to threads

    Principle The general mechanism for executing tasks is the executor service. If you think in terms o ...

  6. Effective Java 18 Prefer interfaces to abstract classes

    Feature Interface Abstract class Defining a type that permits multiple implementations Y Y Permitted ...

  7. Effective Java 20 Prefer class hierarchies to tagged classes

    Disadvantage of tagged classes 1. Verbose (each instance has unnecessary irrelevant fields). 2. Erro ...

  8. Effective Java 49 Prefer primitive types to boxed primitives

    No. Primitives Boxed Primitives 1 Have their own values Have identities distinct from their values 2 ...

  9. Effective Java Index

    Hi guys, I am happy to tell you that I am moving to the open source world. And Java is the 1st langu ...

随机推荐

  1. ok6410 android driver(8)

    In the past, we know how to create and run a simple character device driver on pc, goldfish and ok64 ...

  2. dp - 2015 Multi-University Training Contest 2 1004 Delicious Apples

    Delicious Apples Problem's Link: http://acm.hdu.edu.cn/showproblem.php?pid=5303 Mean: 一条长为L的环形路上种着n棵 ...

  3. 重构第18天 用条件语句来代替异常(Replace exception with conditional)

    理解:本文中的“使用条件判断代替异常”是指把没有必要使用异常做判断的条件尽量改为条件判断. 详解: 重构前代码: public class Microwave { private IMicrowave ...

  4. .Net 自定义应用程序配置

    .Net 自定义应用程序配置 引言 几乎所有的应用程序都离不开配置,有时候我们会将配置信息存在数据库中(例如大家可能常会见到名为Config这样的表):更多时候,我们会将配置写在Web.config或 ...

  5. 微软modern.IE网站,多版本IE免费测试工具集

    微软今天发布了modern.IE,这是一系列免费的.针对Web 开发者的测试工具和资源集合网站,微软希望以此来帮助开发者更轻松地实现跨 IE 和其他现代浏览器.跨设备的兼容性,其他还有代码检测工具.标 ...

  6. bootstrap - table

    http://bootstrap-table.wenzhixin.net.cn/zh-cn/documentation/

  7. 修改RectTransform的值

    用uGUI的时候.经常需要动态改变RectTransform的值,

  8. 【转】 StringUtils中 isNotEmpty 和isNotBlank的区别

    [转自]http://blog.csdn.net/foamflower/article/details/5713604 isNotEmpty将空格也作为参数,isNotBlank则排除空格参数 Str ...

  9. 【Asphyre引擎】发布了新版本V101

    引擎简称还是PXL,但是这个P是Platform而不是Pascal. 修复了一些bug,增加了轻量级的随机数发生器,进一步完善了XML的解析. 不是很明白,为何把Pascal扩展库改成Platform ...

  10. Ahjesus获取自定义属性Attribute或属性的名称

    1:设置自己的自定义属性 public class NameAttribute:Attribute { private string _description; public NameAttribut ...