book

100 Java Mistakes and How to Avoid Them

by Tagir Valeev

May 2024

Intermediate to advanced

352 pages

10h 45m

English

Manning Publications

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1.1 Code review and pair programming1.2 Code style1.3 Static analysis1.3.1 Static analysis tools for Java1.3.2 Using static analyzers1.3.3 Limitations of static analysis1.3.4 Suppressing unwanted warnings1.4 Automated testing1.5 Mutation coverage1.6 Dynamic analysis1.7 Code assertions

2.1 Mistake 1: Incorrect assumptions about numeric operator precedence2.1.1 Binary shift2.1.2 Bitwise operators2.2 Mistake 2: Missing parentheses in conditions2.2.1 && and || precedence2.2.2 Conditional operator and addition2.2.3 The conditional operator and null check2.3 Mistake 3: Accidental concatenation instead of addition2.4 Mistake 4: Multiline string literals2.5 Mistake 5: Unary plus2.6 Mistake 6: Implicit type conversion in conditional expressions2.6.1 Boxed numbers in conditional expressions2.6.2 Nested conditional expressions2.7 Mistake 7: Using non-short-circuit logic operators2.8 Mistake 8: Confusing && and ||2.9 Mistake 9: Incorrectly using variable arity calls2.9.1 Ambiguous variable arity calls2.9.2 Mixing array and collection2.9.3 Using primitive array in variable arity call2.10 Mistake 10: Conditional operators and variable arity calls2.11 Mistake 11: Ignoring an important return value2.12 Mistake 12: Not using a newly created object2.13 Mistake 13: Binding a method reference to the wrong method2.14 Mistake 14: Using the wrong method in a method reference
3.1 Mistake 15: A malformed if–else chain3.2 Mistake 16: A condition dominated by a previous condition3.3 Mistake 17: Accidental pass through in a switch statement3.4 Mistake 18: Malformed classic for loop3.5 Mistake 19: Not using the loop variable3.6 Mistake 20: Wrong loop direction3.7 Mistake 21: Loop overflow3.8 Mistake 22: Idempotent loop body3.9 Mistake 23: Incorrect initialization order3.9.1 Static fields3.9.2 Subclass fields3.9.3 Class initialization order3.9.4 Enum initialization loop3.10 Mistake 24: A missing superclass method call3.11 Mistake 25: Accidental static field declaration
4.1 Mistake 26: Accidental use of octal literal4.2 Mistake 27: Numeric overflow4.2.1 Overflow in Java4.2.2 Assigning the result of int multiplication to a long variable4.2.3 File size, time, and financial computations4.3 Mistake 28: Rounding during integer division4.4 Mistake 29: Absolute value of Integer.MIN_VALUE4.5 Mistake 30: Oddness check and negative numbers4.6 Mistake 31: Widening with precision loss4.7 Mistake 32: Unconditional narrowing4.8 Mistake 33: Negative hexadecimal values4.9 Mistake 34: Implicit type conversion in compound assignments4.10 Mistake 35: Division and compound assignment4.11 Mistake 36: Using the short type4.12 Mistake 37: Manually writing bit-manipulating algorithms4.13 Mistake 38: Forgetting about negative byte values4.14 Mistake 39: Incorrect clamping order4.15 Mistake 40: Misusing special floating-point numbers4.15.1 Signed zero: +0.0 and –0.04.15.2 Not a number: NaN values4.15.3 Double.MIN_VALUE is not the minimal value
5.1 Mistake 41: NullPointerException5.1.1 Avoiding nulls and defensive checks5.1.2 Using Optional instead of null5.1.3 Nullity annotations5.2 Mistake 42: IndexOutOfBoundsException5.3 Mistake 43: ClassCastException5.3.1 Explicit cast5.3.2 Generic types and implicit casts5.3.3 Different class loaders5.4 Mistake 44: StackOverflowError5.4.1 Deep but finite recursion5.4.2 Infinite recursion
6.1 Mistake 45: Assuming that char value is a character6.2 Mistake 46: Unexpected case conversions6.3 Mistake 47: Using String.format with the default locale6.4 Mistake 48: Mismatched format arguments6.5 Mistake 49: Using plain strings instead of regular expressions6.6 Mistake 50: Accidental use of replaceAll6.7 Mistake 51: Accidental use of escape sequences6.8 Mistake 52: Comparing strings in different case6.9 Mistake 53: Not checking the result of indexOf method6.10 Mistake 54: Mixing arguments of indexOf
7.1 Mistake 55: Use of reference equality instead of the equals method7.2 Mistake 56: Assuming equals() compares content7.3 Mistake 57: Using URL.equals()7.4 Mistake 58: Comparing BigDecimals with different scales7.5 Mistake 59: Using equals() on unrelated types7.6 Mistake 60: Malformed equals() implementation7.7 Mistake 61: Wrong hashCode() with array fields7.8 Mistake 62: Mismatch between equals() and hashCode()7.9 Mistake 63: Relying on a particular return value of compare()7.10 Mistake 64: Failing to return 0 when comparing equal objects7.11 Mistake 65: Using subtraction when comparing numbers7.12 Mistake 66: Ignoring possible NaN values in comparison methods7.13 Mistake 67: Failing to represent an object as a sequence of keys in a comparison method7.14 Mistake 68: Returning random numbers from the comparator
8.1 Mistake 69: Searching the object of unrelated type8.2 Mistake 70: Mixing up single object and collection8.3 Mistake 71: Searching for null in a null-hostile collection8.4 Mistake 72: Using null values in maps8.5 Mistake 73: Trying to modify an unmodifiable Collection8.6 Mistake 74: Using mutable objects as keys8.7 Mistake 75: Relying on HashMap and HashSet encounter order8.8 Mistake 76: Concurrent modification during the iteration8.9 Mistake 77: Mixing List.remove() overloads8.10 Mistake 78: Jumping over the next element after List.remove()8.11 Mistake 79: Reading the collection inside Collection.removeIf()8.12 Mistake 80: Concurrent modification in Map.computeIfAbsent()8.13 Mistake 81: Violating Iterator contracts
9.1 Mistake 82: Passing char to StringBuilder constructor9.2 Mistake 83: Producing side effects in a Stream API chain9.3 Mistake 84: Consuming the stream twice9.4 Mistake 85: Using null values in a stream where it’s not allowed9.5 Mistake 86: Violating the contract of Stream API operations9.6 Mistake 87: Using getClass() instead of instanceof9.7 Mistake 88: Using getClass() on enums, annotations, or classes9.8 Mistake 89: Incorrect conversion of string to boolean9.9 Mistake 90: Incorrect format specifiers in date formatting9.10 Mistake 91: Accidental invalidation of weak or soft references9.11 Mistake 92: Assuming the world is stable9.12 Mistake 93: Non-atomic access to concurrently updated data structures
10.1 Mistake 94: Side effect in assert statement10.2 Mistake 95: Malformed assertion method calls10.3 Mistake 96: Malformed exception test10.4 Mistake 97: Premature exit from test method10.5 Mistake 98: Ignoring the AssertionError in unit tests10.6 Mistake 99: Using assertNotEquals() to check the equality contract10.7 Mistake 100: Malformed test methods
A.1 Annotation packagesA.2 Kinds of annotations
B.1 Error Prone pluginsB.2 SpotBugs pluginsB.3 IntelliJ IDEA pluginB.4 Using structural search and replacement in IntelliJ IDEA

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about the author

Tagir Valeev is a technical lead at JetBrains GmbH. He belongs to the Java team, which is responsible for Java language support in IntelliJ IDEA. His primary interest is static analysis and code refactoring. Tagir designed and developed many code inspections for IntelliJ IDEA’s built-in static analyzer. Previously, Tagir contributed to FindBugs, which is a Java bytecode static analysis tool. Tagir has 15 years of experience writing commercial software in Java and 10 years of experience in the field of static analysis. He holds a PhD in computer science (2006) and the title Java Champion (2020).