在java中的长度和长度（） [英] length and length() in java

问题描述

为什么我们有一个数组作为属性的长度。 array.length 和字符串，我们有一个方法 str.length（）？

Why do we have length of an array as an attribute. array.length and for String we have a method str.length()?

刚进来在我心中，是有一些原因是什么？

Just came in my mind, is there some reason?

推荐答案

简化你可以把它看作是数组的特例，而不是普通班（有点像原语，但不是）一个位。字符串和所有的藏品类，因此得到的大小，长度或类似的事情的方法。

A bit simplified you can think of it as arrays being a special case and not ordinary classes (a bit like primitives, but not). String and all the collections are classes, hence the methods to get size, length or similar things.

我想在设计时的原因是表现，如果他们今天创造了它他们可能拿出像数组支持的集合类来代替。

I guess the reason at the time of the design was performance, if they created it today they had probably come up with something like array backed collection classes instead.

如果有人有兴趣，这里是code的一个小片段来说明产生code，两者之间的区别，首先把源代码：

If anyone is interested, here is a small snippet of code to illustrate the difference between the two in generated code, first the source:

public class LengthTest {
  public static void main(String[] args) {
    int[] array = {12,1,4};
    String string = "Hoo";
    System.out.println(array.length);
    System.out.println(string.length());
  }
}

切割方式字节code的不那么重要组成部分，在最后两行以下的类结果运行的javap -c：

Cutting a way the not so important part of the byte code, running javap -c on the class results in the following for the two last lines:

20: getstatic   #3; //Field java/lang/System.out:Ljava/io/PrintStream;
23: aload_1
24: arraylength
25: invokevirtual   #4; //Method java/io/PrintStream.println:(I)V
28: getstatic   #3; //Field java/lang/System.out:Ljava/io/PrintStream;
31: aload_2
32: invokevirtual   #5; //Method java/lang/String.length:()I
35: invokevirtual   #4; //Method java/io/PrintStream.println:(I)V

在第一种情况下（20-25）在code只是要求JVM对数组的大小（JNI这会已经到GetArrayLength（）的调用），而在字符串的情况下（28-35 ），它需要做的方法调用来获取长度。

In the first case (20-25) the code just ask the JVM for the size of the array (in JNI this would have been a call to GetArrayLength()) whereas in the String case (28-35) it needs to do a method call to get the length.

在90年代中期，没有良好的即时编译器之类的东西，它会完全杀死性能只拥有java.util.Vector中（或类似的东西），而不是一门语言结构，它并没有真正像一个类但快速。他们当然可以掩盖的属性作为一个方法调用，并在编译器处理它，但我认为它会一直变得更加令人困惑有某种东西的一种方法，它是不是一个真正的类。

In the mid 90's, without good JITs and stuff, it would have killed performance totally to only have the java.util.Vector (or something similar) and not a language construct which didn't really behave like a class but was fast. They could of course have masked the property as a method call and handled it in the compiler but I think it would have been even more confusing to have a method on something that isn't a real class.

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