Convert two dimensional array of double to string in Java

Question

Is there a simple way to convert double[][] to a String, showing all the values?

Arrays.deepToString does not work, because the inner values are double[], which creates a compilation error:

 incompatible types: double[] cannot be converted to java.lang.Object[]

Arrays.toString will compile, but the resulting string is something like [[D@64c25a62, [D@43e8f1c], which is not very useful.

I think the most concise solution is something like

Arrays.stream(a).
   map(Arrays::toString).
   collect(Collectors.joining("[", "," "]"))

which isn't exactly concise.

Is there something better?

EDIT

Actually, Arrays.deepToString does do what I want. The error I was seeing that I thought mean it didn't work was from trying to pass a double[] to deepToString

Define concise. Your example is concise enough for a JVM to reach the intended result. — queeg
– queeg, Commented Jul 13, 2022 at 16:25
Your error suggests you are attempting to pass a double[] to Arrays#deepToString, but that should be passed to Arrays#toString. A double[][] will be correctly handled by Arrays#deepToString (tested against Java 8 and 17), where it will pass the nested arrays to Array#toString. Can you show an example array you are attempting this with? — Rogue
– Rogue, Commented Jul 13, 2022 at 16:27

Rogue · Accepted Answer · 2022-07-13 16:31:37Z

Note that Arrays#deepToString should only be used for multi-dimensional arrays when dealing with primitives. The error you got was because you attempted to pass a single-dimensional array of primitives, which should instead be sent to Arrays#toString.

double[] one = new double[]{4};
double[][] two = new double[][]{new double[]{1}, new double[]{2}, new double[]{3}};
String res;
res = Arrays.deepToString(two); //Good: #deepToString takes double[][]
res = Arrays.toString(one); //Good: #toString takes double[]
res = Arrays.deepToString(one); //compilation error: #deepToString cannot use double[]

(Above tested on Java 8 and 17).

Per the javadoc of Arrays#deepToString:

If an element e is an array of a primitive type, it is converted to a string as by invoking the appropriate overloading of Arrays.toString(e). If an element e is an array of a reference type, it is converted to a string as by invoking this method recursively.

Jean-Baptiste Yunès · Accepted Answer · 2022-07-13 16:48:58Z

0

Use streams...

Double[][] test = {{1.0,1.2},{2.0,2.1}};
String s = Arrays.stream(test)       // Stream<Double[]>
          .flatMap(Arrays::stream)   // Stream<Double>
          .map(Object::toString)     // Stream<String>
          .collect(Collectors.joining(",", "[", "]"));
System.out.println(s);

flatMap will convert inner array to streams and catenate the streams into a single one.

If you want the same with double then convcert to the appropriate stream:

double[][] test2 = {{1.0,1.2},{2.0,2.1}};
System.out.println(test2);
String s = Arrays.stream(test2)            // Stream<double[]>
          .flatMapToDouble(Arrays::stream) // DoubleStream
          .map(Object::toString)           // Stream<String>
          .collect(Collectors.joining(",", "[", "]"));
System.out.println(s);

edited Jul 13, 2022 at 16:48

answered Jul 13, 2022 at 16:32

Jean-Baptiste Yunès

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