Common Beginner Coding Problems

Note: All example code is in Java.

Operating on literal "true" or "false"

b == false

!b

General Rule: the boolean literals, i.e. the keywords "true" and "false", should only be assigned, passed or returned. Don't operate on them with "==", "!=", "&&", "||" or "!": every such case can be simplified.

Discussion: this is a reason to use good variable names for booleans, e.g. "if (!checked) ..." and "if (isRaining()) ..." read better than "if (x) ...", and make it less tempting to add "== true".

Checking a boolean instead of directly using it

 if (x < 2) {
   return true;
 } else {
   return false;
 }

 return x < 2;

 if (it.next().isRed()) {
   m(true);
   return false;
 } else {
   m(false);
   return true;
 }

 boolean nextIsRed = it.next().isRed();
 m(nextIsRed);
 return !nextIsRed;

General Rule: an if-else with branches differing only in the use of "true" versus "false" should be rewritten without if-else. Use one of the branches, replacing its differing uses of "true" or "false" with the condition or negated condition. If there is more than one use of a boolean literal and the condition is complex or has side-effects, store the condition's value in a temporary variable and use that.

Repeating a universal or optional sub-condition in a boolean expression

(a && b) || (c && a)

a && (b || c)

(a || b) && (a || c)

a || (b && c)

General Rule: undistribute the two Distributive Laws of Propositional Logic, as shown in the examples, "factoring out" the repeated condition.

Exception: the simplification usually isn't equivalent if the repeated expression has side-effects.

Discussion: think about the similarity to the next issue below.

Repeating a statement at the start or end of both branches of an if-else

 if (x()) {
   a();
   b();
   c();
 } else {
   a();
   d();
   c();
 }

// Assumes a() and x() don't have side-effects affecting each other
 a();
 if (x()) {
   b();
 } else {
   d();
 }
 c();

General Rule: move statements appearing at the start or end of both branches of an if-else outside of the if-else.

Exception: the simplification isn't equivalent if the condition and a statement moved from the start of the branches have side-effects affecting each other. In this case consider storing the condition's value in a temporary variable first:

 if (<condition>) {
   <repeated-statement>
   <rest-of-first-branch>
 } else {
   <rest-of-second-branch>
   <statements>
 }

 boolean c = <condition>;
 <repeated-statement>
 if (c) {
   <rest-of-first-branch>
 } else {
   <rest-of-second-branch>
 }

Discussion: think about the similarity to the previous issue above.

Mutually exclusive conditions checked by nesting if inside an else

 if (x < 0) {
   s1();
 } else {
   if (x == 0) {
     s2();
   }
 }

 if (x < 0) {
   s1();
 } else if (x == 0) {
   s2();
 }

General Rule: use the unnested form of if-else-if for mutually exclusive or related conditions. In Java this means using the unbraced form for the else clause, and unindenting one level. In languages with a dedicated special syntax for this case, use that: discussion of convention versus special syntax.

Exception: you have considered both forms, and there levels of condition checking you want to emphasize. For example:

  if (x < 0) {
    if (y == 1) {
      s1();
    } else if (y == 3) {
      s3();
    }
  } else {
    // condition on y relying on implicit x >= 0
    if (y == 2) {
      s2();
    }
  }

Declaring then initializing a variable as two consecutive statements

 int x;
 x = 0;

 int x = 0;

General Rule: don't declare and initialize a variable in two consecutive statements, instead use the single statement form.

Exception: two or more related variables are declared together but only some are initialized. For example:

  int x, y;
  y = 0;