[Java 7] Divide the Java list and execute the process

Divide Java list and execute processing (Java 7)

Introduction

Depending on the environment, you may have to write old code on purpose. I had the opportunity to write code in Java7 that divides the List and executes the process, so I posted it for the first time.

Implementation

public class Sample {
	public static void main(String[] args) {

		List<String> sampleList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");
		int size = 2;	//Number of lists to process at one time

		for (int i = 0; i < sampleList.size(); i += size) {
			List<String> list = new ArrayList<>(
					sampleList.subList(i, Math.min(i + size, sampleList.size()))
			);
			System.out.println(list);	//Perform processing here
		}
	}
}

Execution result

[aaa, bbb]	// sampleList.subList(0, 2)
[ccc, ddd]	// sampleList.subList(2, 4)
[eee]		// sampleList.subList(4, 5)

Description

subList (int from, int to) returns a List from from (included) to to (not included). The list to be processed is acquired by increasing the number of divisions of from and to by the for statement.

Math.min (int a, int b) returns the smaller of the two int values. If the size of the List is not divisible by the number of divisions, then you need to consider to, For the last iteration, we put sampleList.size () in to.

Notes on subList

The List obtained by subList shares data with the original value. That is, changing the value of list will change the value of sampleList (Example 1). However, you can change only the subList without changing the original List by making changes after new (Example 2).

Example 1 When the subList is changed, the original list is also changed.

public class Sample {
	public static void main(String[] args) {

		List<String> sampleList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");
		int size = 2;	//Number of lists to process at one time

		System.out.println(sampleList);		//Before processing
		for (int i = 0; i < sampleList.size(); i += size) {
			List<String> list = sampleList.subList(i, Math.min(i + size, sampleList.size()));
			list.set(0, "zzz");				//Processing execution
			System.out.println(list);
		}
		System.out.println(sampleList);		//After treatment
	}
}

Execution result

[aaa, bbb, ccc, ddd, eee]	//Before processing
[zzz, bbb]
[zzz, ddd]
[zzz]
[zzz, bbb, zzz, ddd, zzz]	//After treatment

As mentioned above, as a result of updating list, it can be seen that sampleList has also been updated.

Example 2 By new, the original list is not updated even if the subList is changed.

public class Sample {
	public static void main(String[] args) {

		List<String> sampleList = Arrays.asList("aaa", "bbb", "ccc", "ddd", "eee");
		int size = 2;	//Number of lists to process at one time

		System.out.println(sampleList);		//Before processing
		for (int i = 0; i < sampleList.size(); i += size) {
			List<String> list = new ArrayList<>(
					sampleList.subList(i, Math.min(i + size, sampleList.size()))
			);
			list.set(0, "zzz");
			System.out.println(list);
		}
		System.out.println(sampleList);		//After treatment
	}
}

Execution result

[aaa, bbb, ccc, ddd, eee]	//After treatment
[zzz, bbb]
[zzz, ddd]
[zzz]
[aaa, bbb, ccc, ddd, eee]	//After treatment

As mentioned above, you can see that changing list does not change sampleList.

reference

• Math (Java Platform SE 7) - Oracle Docs
• List (Java Platform SE 7) - Oracle Docs -Process each arbitrary number in Java List-Qiita