Java | 配列と List の複雑ネストをまとめた ASCII 図＋出力例付きの一覧

では、配列と List の複雑ネストをまとめた ASCII 図＋出力例付きの一覧を作ります。これで「多次元・ネスト構造での println / toString / deepToString の挙動」が一目でわかるようになります。

1. ネスト構造一覧
2. ASCII 図でイメージ化
3. コピペで確認コード
💡 ポイント総まとめ

1. ネスト構造一覧

構造	変数例	println	toString	deepToString / 個別表示
1次元配列	`int[] one = {1,2,3}`	`[I@xxxxxx`	`[1,2,3]`	`[1,2,3]`
2次元配列	`int[][] two = {{10,20},{30,40}}`	`[I@xxxx, [I@yyyy]`	`[ [I@xxxx, [I@yyyy] ]`	`[[10,20],[30,40]]`
null 配列	`int[] a = null`	`null`	NPE	`[null]` (Object 配列でラップ)
配列の一部表示	`int[] big` → `Arrays.copyOfRange(big,0,5)`	–	`[1,2,3,4,5]`	`[1,2,3,4,5]`
配列の中に List	`Object[] arrayWithList = {Arrays.asList(1,2), Arrays.asList(3,4)}`	`[Ljava.lang.Object;@xxxx`	`[[1,2],[3,4]]`	`[[1,2],[3,4]]`
List の中に配列	`List<Object> listWithArray = Arrays.asList(new int[]{5,6}, new int[]{7,8})`	`[[I@xxxx, [I@yyyy]]`	–	個別に `Arrays.toString` → `[5,6]` `[7,8]`

2. ASCII 図でイメージ化

1次元配列
one: [1,2,3]
      ^ ^ ^
     val val val
println → REF
toString → [1,2,3]
deepToString → [1,2,3]

2次元配列
two: [ row0 , row1 ]
       |      |
       v      v
    row0: [10,20]
    row1: [30,40]
println → REF
toString → [REF_row0, REF_row1]
deepToString → [[10,20],[30,40]]

配列の中に List
arrayWithList: [ list0 , list1 ]
                 |       |
                 v       v
             list0: [1,2]
             list1: [3,4]
println → REF
toString → [[1,2],[3,4]]
deepToString → [[1,2],[3,4]]

List の中に配列
listWithArray: [ arr0 , arr1 ]
                 |       |
                 v       v
             arr0: [5,6]
             arr1: [7,8]
println → [[I@xxxx, [I@yyyy]]
個別に toString → [5,6], [7,8]

null 配列
a: null
println → null
toString → NPE
deepToString → [null]

部分表示
big: [1,2,3,4,5,6,7,8,9,10]
firstN = copyOfRange(big,0,5)
Arrays.toString(firstN) → [1,2,3,4,5]

3. コピペで確認コード

import java.util.Arrays;
import java.util.List;

public class FullNestedDemo {
    public static void main(String[] args) {
        int[] one = {1,2,3};
        int[][] two = {{10,20},{30,40}};
        int[] nullArray = null;
        int[] big = {1,2,3,4,5,6,7,8,9,10};
        Object[] arrayWithList = { Arrays.asList(1,2), Arrays.asList(3,4) };
        List<Object> listWithArray = Arrays.asList(new int[]{5,6}, new int[]{7,8});

        System.out.println("1次元配列 println: " + one);
        System.out.println("1次元配列 toString: " + Arrays.toString(one));
        System.out.println("1次元配列 deepToString: " + Arrays.deepToString(new Object[]{one}));
        System.out.println();

        System.out.println("2次元配列 println: " + two);
        System.out.println("2次元配列 toString: " + Arrays.toString(two));
        System.out.println("2次元配列 deepToString: " + Arrays.deepToString(two));
        System.out.println();

        System.out.println("null 配列 println: " + nullArray);
        System.out.println("null 配列 deepToString: " + Arrays.deepToString(new Object[]{nullArray}));
        System.out.println();

        int[] firstN = Arrays.copyOfRange(big, 0, 5);
        System.out.println("部分表示: " + Arrays.toString(firstN));
        System.out.println();

        System.out.println("配列の中に List:");
        System.out.println("println: " + arrayWithList);
        System.out.println("toString: " + Arrays.toString(arrayWithList));
        System.out.println("deepToString: " + Arrays.deepToString(arrayWithList));
        System.out.println();

        System.out.println("List の中に配列:");
        System.out.println("println: " + listWithArray);
        System.out.println("個別に toStringで中身表示:");
        listWithArray.stream()
            .map(o -> Arrays.toString((int[])o))
            .forEach(System.out::println);
    }
}

import java.util.Arrays;
import java.util.List;

public class FullNestedDemo {
    public static void main(String[] args) {
        int[] one = {1,2,3};
        int[][] two = {{10,20},{30,40}};
        int[] nullArray = null;
        int[] big = {1,2,3,4,5,6,7,8,9,10};
        Object[] arrayWithList = { Arrays.asList(1,2), Arrays.asList(3,4) };
        List<Object> listWithArray = Arrays.asList(new int[]{5,6}, new int[]{7,8});

        System.out.println("1次元配列 println: " + one);
        System.out.println("1次元配列 toString: " + Arrays.toString(one));
        System.out.println("1次元配列 deepToString: " + Arrays.deepToString(new Object[]{one}));
        System.out.println();

        System.out.println("2次元配列 println: " + two);
        System.out.println("2次元配列 toString: " + Arrays.toString(two));
        System.out.println("2次元配列 deepToString: " + Arrays.deepToString(two));
        System.out.println();

        System.out.println("null 配列 println: " + nullArray);
        System.out.println("null 配列 deepToString: " + Arrays.deepToString(new Object[]{nullArray}));
        System.out.println();

        int[] firstN = Arrays.copyOfRange(big, 0, 5);
        System.out.println("部分表示: " + Arrays.toString(firstN));
        System.out.println();

        System.out.println("配列の中に List:");
        System.out.println("println: " + arrayWithList);
        System.out.println("toString: " + Arrays.toString(arrayWithList));
        System.out.println("deepToString: " + Arrays.deepToString(arrayWithList));
        System.out.println();

        System.out.println("List の中に配列:");
        System.out.println("println: " + listWithArray);
        System.out.println("個別に toStringで中身表示:");
        listWithArray.stream()
            .map(o -> Arrays.toString((int[])o))
            .forEach(System.out::println);
    }
}