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These methods are often used to read a byte or character from a stream. Unfortunately, many programmers prematurely convert the resulting int back to a byte or char before checking whether they have reached the end of the stream (signaled indicated by a return value of -1). Programs must check for the end of stream (e.g., -1) before narrowing the return value to a byte or char.

This rule applies to any InputStream or Reader subclass that provide provides an implementation of the read() method. This rule is a specific instance of rule NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data.

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This noncompliant code example casts the value returned by the read() method directly to a value of type byte and then compares this value with -1 in an attempt to detect the end of the stream. This conversion leaves the value of c as 0xFFFF (e.g., Character.MAX_VALUE) instead of -1. Consequently, the test for the end of stream never evaluates to true (because the char type is unsigned and the value of c is 0-extended to 0x0000FFFF).

FileInputStream in;
// initialize stream 
byte data;
while ((data = (byte) in.read()) != -1) { 
  // ... 
}

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Use a variable of type int to capture the return value of the byte input method. When the value returned by read() is not -1, it can be safely cast to type byte. When read() returns 0x000000FF, the comparison will test against 0xFFFFFFFF, which evaluates to false.

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This noncompliant code example casts the value of type int returned by the read() method directly to a value of type char, which is then compared with -1 in an attempt to detect the end of stream. This conversion leaves the value of c as 0xffff 0xFFFF (e.g., Character.MAX_VALUE) instead of -1. Consequently, the test for the end of stream never evaluates to true (because char type is unsigned and the value of c is 0-extended to 0x0000FFFF).

FileReader in;
// initialize stream 
char c;
while ((c = (char) in.read()) != -1) { 
  // ... 
}

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