Web 0.14.21: Array-Ausgabe wie in Java (Referenz), Arrays.toString zeigt den Inhalt (#166)

System.out.println(zahlen) gibt in der Simulation und in Programmieren jetzt wie
echtes Java eine Referenz wie [I@… aus. Arrays.toString und Arrays.deepToString
zeigen den Inhalt; Arrays braucht import java.util.Arrays (Code-Export ergänzt ihn).
Lektion 15 erklärt das in de/en/pt.


Claude-Session: https://claude.ai/code/session_01Ugz28VVb2pkKNmmHKgsfxu

Co-authored-by: Claude <noreply@anthropic.com>

Author: code4teaching

CHANGELOG.md

@@ -8,6 +8,7 @@ The format is loosely inspired by Keep a Changelog.
 
 ### Changed
 
+- **Web 0.14.21, array output like in Java:** `System.out.println(zahlen)` now prints a reference such as `[I@1e3779b1` in Simulation and Programmieren, as real Java does (`[D@`, `[Z@`, `[Ljava.lang.String;@`, `[[I@`; the same array keeps its id). `Arrays.toString(…)` and `Arrays.deepToString(…)` show the content; `Arrays` needs `import java.util.Arrays;` like javac, and the Java export adds it. The variable table still shows the content. Lesson 15 „Jetzt in Java“ explains reference and `Arrays.toString` (de/en/pt).
 - **Web 0.14.18, upper and lower case, exercises „Warum läuft das nicht?“:** lesson 4 gets the new step „Groß- und Kleinschreibung“ (case-sensitive: `zahl`, `Zahl` and `ZAHL` are three names) with a structure chart exercise (`zahl` and `Zahl` both declared, `ZAHL` used), a Kompetenz item and a quiz question; glossary entry case-sensitive. Lessons 4–17 and 19 each get a Java exercise „Warum läuft das nicht?“: the start code doesn't run (misspelled variable such as `Alter`/`Grenze`/`properson`, variable used before it is created or never created, `system.out`, `.Length`, `charat`), the student reads the error message, fixes it and clicks Check (de/en/pt). The Code area now reports wrong case in Java words (`system.out.println`, `System.Out`, `Println`, `math.sqrt`, `text.Length()`, `name.charat(0)`) with the right spelling, blocks Run like javac and offers the 💡 fix; the simulation says „Unbekannte Variable: Zahl. Java unterscheidet Groß- und Kleinschreibung: Meintest du zahl?“ when the name exists in another case (all 21 languages).
 - **Web 0.14.17, hint on a missing line break:** when a Java exercise's output is right except that lines run together (e.g. `System.out.print` twice instead of `println`), the check now says „Fast! Es fehlt nur ein Zeilenumbruch …“ and explains print vs. println, instead of only showing a mismatch. `\n` inside an output now starts a new line, so `System.out.print("…\n")` counts like `println` (de/en/pt; all exercises).
 - **Web 0.14.16, „Im Editor öffnen“ under the Java examples in lessons 4–19:** every runnable Java example in the lesson text of lessons 4–19 now has the button „Im Editor öffnen“ (like the Mia example in lesson 4), so students can run and change it in Programmieren. Not in lesson 18, whose examples are a method and its call on their own, and not in cloze texts or games (de/en/pt).

web/lessons/de/15-arrays.md

@@ -166,6 +166,8 @@ for (int i = 0; i < zahlen.length; i++) {
 - Ein leeres Array mit 5 Fächern legst du mit `new int[5]` an.
 - `zahlen[i]` ist das Fach mit dem Index `i`, `zahlen.length` die Länge.
 
+Gibst du das ganze Array aus, also `System.out.println(zahlen);`, zeigt Java nicht die Zahlen. Du siehst nur einen Text wie `[I@1b6d3586`. Das ist die **Referenz**, also die Stelle, an der das Array im Speicher liegt. Den Inhalt zeigt erst `Arrays.toString(zahlen)`, das ergibt `[2, 4, 6, 8, 10]`. Dafür schreibst du ganz oben `import java.util.Arrays;`.
+
 ::: übung l15-java-noten
 titel: Notensumme in Java
 code: java

web/lessons/en/15-arrays.md

@@ -166,6 +166,8 @@ for (int i = 0; i < numbers.length; i++) {
 - You create an empty array with 5 slots with `new int[5]`.
 - `numbers[i]` is the slot with index `i`, `numbers.length` is the length.
 
+If you output the whole array, that is `System.out.println(numbers);`, Java does not show the numbers. You only see a text like `[I@1b6d3586`. This is the **reference**, the place where the array is stored in memory. `Arrays.toString(numbers)` shows the content: `[2, 4, 6, 8, 10]`. For this you write `import java.util.Arrays;` at the very top.
+
 ::: übung l15-java-noten
 titel: Sum of grades in Java
 code: java

web/lessons/pt/15-arrays.md

@@ -166,6 +166,8 @@ for (int i = 0; i < numeros.length; i++) {
 - Um array vazio com 5 posições cria-se com `new int[5]`.
 - `numeros[i]` é a posição com o índice `i`, `numeros.length` é o comprimento.
 
+Se mostrares o array inteiro, ou seja `System.out.println(numeros);`, o Java não mostra os números. Só vês um texto como `[I@1b6d3586`. É a **referência**, o sítio onde o array está guardado na memória. O conteúdo só aparece com `Arrays.toString(numeros)`, que dá `[2, 4, 6, 8, 10]`. Para isso escreves `import java.util.Arrays;` logo no início.
+
 ::: übung l15-java-noten
 titel: Soma das notas em Java
 code: java

web/package-lock.json

@@ -1,7 +1,7 @@
 {
   "name": "visustruct-web",
   "private": true,
-  "version": "0.14.20",
+  "version": "0.14.21",
   "type": "module",
   "scripts": {
     "dev": "vite",

web/package.json

@@ -1095,27 +1095,31 @@ function documentContainsJavaInput(doc: VSDocument): boolean {
   return found
 }
 
+const arraysImport = 'import java.util.Arrays;'
+
 /**
  * Imports für Dateien: `Files`/`Path` aus `java.nio.file`, `File`, `PrintWriter`, `FileReader` und `BufferedReader` aus
- * `java.io`, dazu `java.util.Scanner` für einen Scanner auf einer Datei.
+ * `java.io`, dazu `java.util.Scanner` für einen Scanner auf einer Datei und `java.util.Arrays` für `Arrays.toString(…)`.
  */
 export function fileImports(doc: VSDocument): string[] {
   let nio = false
   let io = false
   let scanner = false
+  let arrays = false
   walkElements(doc.elements, (e) => {
     for (const l of e.textLines) {
+      if (/\bArrays\s*\./.test(l)) arrays = true
       if (/\bFiles\s*\./.test(l)) nio = true
       if (/\bnew\s+(?:File|PrintWriter|FileReader|FileWriter|BufferedReader)\s*\(/.test(l)) io = true
       if (/\bnew\s+Scanner\s*\(\s*new\s+File\b/.test(l)) scanner = true
     }
   })
-  return [...(io ? ['import java.io.*;'] : []), ...(nio ? ['import java.nio.file.*;'] : []), ...(scanner ? ['import java.util.Scanner;'] : [])]
+  return [...(io ? ['import java.io.*;'] : []), ...(nio ? ['import java.nio.file.*;'] : []), ...(scanner ? ['import java.util.Scanner;'] : []), ...(arrays ? [arraysImport] : [])]
 }
 
 /** Arbeitet das Programm mit Dateien? Dann kann eine IOException kommen, und Java verlangt `throws` an der Methode. */
 export function documentReadsFiles(doc: VSDocument): boolean {
-  return fileImports(doc).length > 0
+  return fileImports(doc).some((i) => i !== arraysImport)
 }
 
 function documentContainsJavaMathRandom(doc: VSDocument): boolean {
@@ -1804,7 +1808,7 @@ export function generateJavaClassFile(
   const scannerField = methods.length > 0 && (hasInput || methodInput)
   // `Files.readString` kann eine IOException werfen; Java verlangt dann `throws` an der Methode.
   const imports = [...new Set([document, ...methodDocs.map((m) => m.d)].flatMap(fileImports))]
-  const files = imports.length > 0
+  const files = imports.some((i) => i !== arraysImport)
   if ((hasInput || methodInput) && !imports.includes('import java.util.Scanner;')) imports.push('import java.util.Scanner;')
   for (const line of imports) out += line + '\n'
   if (imports.length) out += '\n'

web/src/core/codeGenerator.ts

@@ -227,14 +227,15 @@ class Parser {
 
   /**
    * Wie javac: `Scanner` ohne `import java.util.Scanner;` (oder `java.util.*`) ist ein Fehler, ebenso `Files` und `Path` ohne
-   * `java.nio.file` und `File`, `PrintWriter`, `FileReader`, `FileWriter`, `BufferedReader` ohne `java.io`.
+   * `java.nio.file`, `File`, `PrintWriter`, `FileReader`, `FileWriter`, `BufferedReader` ohne `java.io` und `Arrays` ohne
+   * `java.util.Arrays`.
    */
   checkImports() {
     const io = ['File', 'PrintWriter', 'FileReader', 'FileWriter', 'BufferedReader', 'IOException', 'FileNotFoundException'].map((n) => [n, 'java.io'])
-    for (const [name, pkg] of [['Scanner', 'java.util'], ['Files', 'java.nio.file'], ['Path', 'java.nio.file'], ['Paths', 'java.nio.file'], ...io]) {
+    for (const [name, pkg] of [['Scanner', 'java.util'], ['Arrays', 'java.util'], ['Files', 'java.nio.file'], ['Path', 'java.nio.file'], ['Paths', 'java.nio.file'], ...io]) {
       const known = this.imports.some((i) => i === `import${pkg}.${name};` || i === `import${pkg}.*;`)
       // `Files.readString(…)`: nur als Klasse vor einem Punkt, damit eine Variable `path` o. Ä. nicht zählt.
-      const use = this.toks.find((t, k) => t.kind === 'id' && t.text === name && (pkg !== 'java.nio.file' || this.toks[k + 1]?.text === '.'))
+      const use = this.toks.find((t, k) => t.kind === 'id' && t.text === name && (name !== 'Arrays' && pkg !== 'java.nio.file' || this.toks[k + 1]?.text === '.'))
       if (!known && use) {
         this.problem('web_parse_missing_import', use, pkg === 'java.util' ? `import ${pkg}.${name};` : `import ${pkg}.*;`)
         return

web/src/core/javaParser.ts

@@ -245,6 +245,38 @@ function elementDefault(type: SimValueType): number | string | boolean {
   return type === 'string' ? '' : type === 'boolean' ? false : 0
 }
 
+/**
+ * Was `System.out.println(zahlen)` in Java ausgibt: nicht den Inhalt, sondern Typ und Kennung der Referenz,
+ * z. B. `[I@1b6d3586`. Den Inhalt zeigt erst `Arrays.toString(zahlen)`. Die Kennung bleibt für dasselbe Array gleich.
+ */
+const arrayIds = new WeakMap<object, string>()
+let nextArrayId = 0
+
+export function javaReference(v: SimValue): string {
+  const code = (t: SimValueType) => ({ int: 'I', double: 'D', boolean: 'Z', string: 'Ljava.lang.String;' })[t]
+  const prefix =
+    v.kind === 'grid' ? '[[' + code(v.elementType)
+    : v.kind === 'intArray' ? '[I'
+    : v.kind === 'doubleArray' ? '[D'
+    : v.kind === 'booleanArray' ? '[Z'
+    : v.kind === 'stringArray' ? '[Ljava.lang.String;'
+    : undefined
+  if (!prefix || !('value' in v)) return describeValue(v)
+  const key = v.value as object
+  let id = arrayIds.get(key)
+  if (!id) {
+    id = ((Math.imul(++nextArrayId, 0x9e3779b1) >>> 0) & 0x7fffffff).toString(16)
+    arrayIds.set(key, id)
+  }
+  return prefix + '@' + id
+}
+
+/** Ein Fach wurde geändert und das Array dabei kopiert: Es ist in Java aber dasselbe Array, also dieselbe Kennung. */
+function keepArrayId(from: object, to: object): void {
+  const id = arrayIds.get(from)
+  if (id) arrayIds.set(to, id)
+}
+
 function isArrayValue(v: SimValue): boolean {
   return v.kind === 'intArray' || v.kind === 'doubleArray' || v.kind === 'stringArray' || v.kind === 'booleanArray' || v.kind === 'grid'
 }
@@ -1938,7 +1970,10 @@ export class SimulationEngine {
     }
   }
 
-  /** `Integer.parseInt(…)`, `Double.parseDouble(…)`, `String.valueOf(…)`, `Files.readString(Path.of(…))`. */
+  /**
+   * `Integer.parseInt(…)`, `Double.parseDouble(…)`, `String.valueOf(…)`, `Files.readString(Path.of(…))`,
+   * `Arrays.toString(…)` und `Arrays.deepToString(…)`.
+   */
   private evaluateStaticCall(text: string, call: { target: string; method: string; args: string[] }): SimValue | undefined {
     const one = () => {
       if (call.args.length !== 1) throw invalid(text)
@@ -1964,6 +1999,17 @@ export class SimulationEngine {
       }
       case 'String.valueOf':
         return { kind: 'string', value: this.evaluateOutputExpression(one()) }
+      case 'Arrays.toString':
+      case 'Arrays.deepToString': {
+        const array = this.state.variables.get(one().trim())
+        if (!array || !isArrayValue(array)) throw invalid(text)
+        // `Arrays.toString` zeigt bei einem 2D-Array nur die Referenzen der Zeilen, `deepToString` alles.
+        if (array.kind === 'grid' && call.method === 'toString') {
+          const rows = array.value.map((row) => javaReference({ kind: `${array.elementType}Array`, value: row } as SimValue))
+          return { kind: 'string', value: '[' + rows.join(', ') + ']' }
+        }
+        return { kind: 'string', value: describeValue(array) }
+      }
       case 'Files.readString': {
         const path = /^(?:Path\s*\.\s*of|Paths\s*\.\s*get)\s*\(\s*("(?:[^"\\]|\\.)*")\s*\)$/u.exec(one().trim())
         if (!path) throw invalid(text)
@@ -2076,6 +2122,7 @@ export class SimulationEngine {
       if (raw === undefined || isArrayValue(value)) throw invalid(trimmed)
       const rows = old.value.map((row) => [...row])
       rows[indices[0]][indices[1]] = raw as number | string | boolean
+      keepArrayId(old.value, rows)
       this.state.variables.set(access.arrayName, { ...old, value: rows })
       return
     }
@@ -2095,6 +2142,7 @@ export class SimulationEngine {
     }
     if (index >= (updated.value as unknown[]).length) throw invalid(trimmed)
     ;(updated.value as unknown[])[index] = value.value
+    keepArrayId(old.value, updated.value)
     this.state.variables.set(access.arrayName, updated)
   }
 
@@ -2122,7 +2170,8 @@ export class SimulationEngine {
         else sum = sum.kind === 'int' && value.kind === 'int' ? int(sum.value + value.value) : dbl((sum as { value: number }).value + (value as { value: number }).value)
         continue
       }
-      text = (text ?? (sum ? describeValue(sum) : '')) + describeValue(value)
+      // Ein ganzes Array gibt Java nur als Referenz aus, z. B. `[I@1b6d3586`.
+      text = (text ?? (sum ? describeValue(sum) : '')) + javaReference(value)
       sum = undefined
     }
     return text ?? (sum ? describeValue(sum) : '')

web/src/core/simulation.ts

@@ -214,6 +214,13 @@ describe('Java tippen → Struktogramm', () => {
     expect(ask(fach + 'note[i] = scanner.nextInt();').request?.prompt).toBe('note[i]:')
   })
 
+  it('Arrays ohne import ist ein Fehler, wie bei javac', () => {
+    const keys = (src: string) => parse(src).problems.map((p) => p.key)
+    expect(keys('int[] a = {1, 2};\nSystem.out.println(Arrays.toString(a));')).toEqual(['web_parse_missing_import'])
+    expect(keys('import java.util.Arrays;\nint[] a = {1, 2};\nSystem.out.println(Arrays.toString(a));')).toEqual([])
+    expect(keys('int[] a = {1, 2};\nSystem.out.println(a);')).toEqual([])
+  })
+
   it('Aufruf eines Unterprogramms aus einem anderen Reiter', () => {
     expect(parse('int y = quadrat(4);', new Set(['quadrat'])).elements[0].type).toBe(ElementType.aufruf)
     expect(parse('int y = quadrat(4);').elements[0].type).toBe(ElementType.anweisung)