I'm having a problem initializing array of HashSet
int N = 100;
HashSet<Integer> []array = new HashSet[N];
for (HashSet<Integer> set:array){
set = new HashSet<Integer>();
}
But the array contains only null. (Also error when HashSet []array = .... )
But when running with:
for(int i = 0; i < N; i++){
array[i] = new HashSet<Integer>();
}
All is well.
Why does the first code isn't working? Is it my mistake?
Thank you
You never actually assign initialized instances to elements of the array. Instead you iterate over the elements of the array with a variable that gets assigned to a new object in your loop, then is never used. In a case like this the enhanced for...each syntax is not appropriate, use the traditional for loop instead.
two 'for' is different in Java, look the java code & bytecode .. Example:
public class T{
public static void main(String[] args){
String[] data = new String[10];
System.out.print("");
for(String str:data){
str="1";
}
System.out.print("");
for(int i=0;i<data.length;i++){
data[i]="1";
}
}
}
$>javac -classpath . T.java
$>javap -c T
Compiled from "T.java"
public class T extends java.lang.Object{
public T();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: bipush 10
2: anewarray #2; //class java/lang/String
5: astore_1
6: getstatic #3; //Field java/lang/System.out:Ljava/io/PrintStream;
9: ldc #4; //String
11: invokevirtual #5; //Method java/io/PrintStream.print:(Ljava/lang/String;)V
14: aload_1
15: astore_2
16: aload_2
17: arraylength
18: istore_3
19: iconst_0
20: istore 4
22: iload 4
24: iload_3
25: if_icmpge 44
28: aload_2
29: iload 4
31: aaload
32: astore 5
34: ldc #6; //String 1
36: astore 5
38: iinc 4, 1
41: goto 22
44: getstatic #3; //Field java/lang/System.out:Ljava/io/PrintStream;
47: ldc #4; //String
49: invokevirtual #5; //Method java/io/PrintStream.print:(Ljava/lang/String;)V
52: iconst_0
53: istore_2
54: iload_2
55: aload_1
56: arraylength
57: if_icmpge 71
60: aload_1
61: iload_2
62: ldc #6; //String 1
64: aastore
65: iinc 2, 1
68: goto 54
71: return
}
from line 25--44 and line 57--71: aload : Retrieves an object reference from a local variable and pushes it onto the operand stack. aaload : Retrieves an object reference from an array of objects and places it on the stack.
astore : Take object or reference store to local variable. aastore : Take reference type value store to array.
so,first can't store array , didn't using initializing array.
An enhanced for loop, does not use the actual instances in the array( and as well, collection), but rather copies them in the loop control variable.
This should not be a problem with non-null values, since they point to the same object. Problems arise, if the values are null, and re-assigning values to the control variable, would not change the actual value. So in this case, always use regular for loops.
The same would be true, to arrays of primitive types, since coping them, and changing the copied variable, will not affect the original variable.
Simply use new Set[n] instead of new HashSet[n] , to get it working
Set<Integer>[] list = new Set[n];
for(int i=0;i<n;i++)
list[i] = new HashSet<>();
You need to create an array that contains HashSet so, you need to initialize the array and then initialize the HashSet which I have provided in the below code
int N = 100;
Set<Integer>[] array = new Set[n];
for(int i = 0; i < n; i++) {
array[i] = new HashSet<Integer>();
}
