问题
How come
int alone;
System.out.println(alone);
Gives errors but
int[] arr = new int[1];
System.out.println(arr[0]);
Equals 0? When you initialize an empty array, does it automatically initialize its contents to 0 (or null, etc.)?
回答1:
From the language standard
Otherwise, a one-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.12.5).
And the default values are:
For type byte, the default value is zero, that is, the value of (byte)0.
For type short, the default value is zero, that is, the value of (short)0.
For type int, the default value is zero, that is, 0.
For type long, the default value is zero, that is, 0L.
For type float, the default value is positive zero, that is, 0.0f.
For type double, the default value is positive zero, that is, 0.0d.
For type char, the default value is the null character, that is, '\u0000'.
For type boolean, the default value is false.
For all reference types (§4.3), the default value is null.
回答2:
Yes, for primitive types(except boolean and char) it will be default to ZERO. If object type it will be default to null.
This java tutorial may help you.
NOTE: as woot4Moo answered, this is for only instance variables. If local variable, then it won't be default to any.
回答3:
Yes, for primitive type number arrays, it initializes with 0, for boolean[], it initializes with false, for char[], it initializes with NULL(ASCII value 0) and for objects [] including String[] its initializes with null.
回答4:
There are two separate by similar issues involved here. First note that all variable types have a default value, which differs depending on the type. There are at least two times where these defaults are used: 1) declaring a member variable and 2) initializing an array with the new operator.
Notice that if you simply declare a local array variable without initializing it with new, then you get the same error as when you declare a simple int variable. This is because all local variables must be initialized. They do not get an automatic default value.
On the other hand, member variables do get a default value. Similarly, when you create an array object by using the new operator, the elements of the array are initialized to the appropriate default value.
回答5:
It depends on where it was declared (inside a class vice inside a function). If it is a class member variable it will be initialized to its default. 0 for numeric types(0.0 for float types / double), null for Strings, false for boolean,and null for Objects. If it is declared inside of a function it would remain uninitialized in the case of int alone. In terms of an array it will always initialize the values contained within it.
来源：https://stackoverflow.com/questions/13167153/empty-arrays-automatically-initialize-contents