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Implicit conversion from array to string

ID: java/print-array
Kind: problem
Security severity: 
Severity: recommendation
Precision: very-high
Tags:
   - maintainability
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

Printing an array is likely to produce unintended results. That is, the result does not contain the contents of the array. This is because the array is implicitly converted to a String using Object.toString, which just returns the following value:

getClass().getName() + '@' + Integer.toHexString(hashCode())

Recommendation

When converting an array to a readable string, use Arrays.toString for one-dimensional arrays, or Arrays.deepToString for multi-dimensional arrays. These functions iterate over the contents of the array and produce human-readable output.

Example

In the following example, the contents of the array words are printed out only if Arrays.toString is called on the array first. Similarly, the contents of the multi-dimensional array wordMatrix are printed out only if Arrays.deepToString is called on the array first.

public static void main(String args[]) {
	String[] words = {"Who", "is", "John", "Galt"};
	String[][] wordMatrix = {{"There", "is"}, {"no", "spoon"}};
	
	// BAD: This implicitly uses 'Object.toString' to convert the contents
	// of 'words[]', and prints out something similar to:
	// [Ljava.lang.String;@459189e1
	System.out.println(words);
	
	// GOOD: 'Arrays.toString' calls 'toString' on
	// each of the array's elements. The statement prints out:
	// [Who, is, John, Galt]
	System.out.println(Arrays.toString(words));
	
	// ALMOST RIGHT: This calls 'toString' on each of the multi-dimensional 
	// array's elements. However, because the elements are arrays, the statement
	// prints out something similar to:
	// [[Ljava.lang.String;@55f33675, [Ljava.lang.String;@527c6768]]
	System.out.println(Arrays.toString(wordMatrix));
	
	// GOOD: This properly prints out the contents of the multi-dimensional array:
	// [[There, is], [no, spoon]]
	System.out.println(Arrays.deepToString(wordMatrix));
}

References

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