BadShiftAmount
Shift by an amount that is out of range
Category JDK
Severity ERROR
Maturity
The problem
For shift operations on int types, only the five lowest-order bits of the shift
amount are used as the shift distance. This means that shift amounts that are
not in the range 0 to 31, inclusive, are silently mapped to values in that
range. For example, a shift of an int by 32 is equivalent to shifting by 0,
i.e., a no-op.

See JLS 15.19, “Shift Operators”, for more details.

Suppression
Suppress false positives by adding an `@SuppressWarnings("BadShiftAmount")`

annotation to the enclosing element.

Positive examples
BadShiftAmountPositiveCases.java

/*
* Copyright 2013 Google Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com . google . errorprone . bugpatterns . testdata ;
/**
* @author Bill Pugh (bill.pugh@gmail.com)
*/
public class BadShiftAmountPositiveCases {
public void foo () {
int x = 0 ;
long result = 0 ;
// BUG: Diagnostic contains: (long) x >> 32
result += x >> 32 ;
// BUG: Diagnostic contains: (long) x << 32
result += x << 32 ;
// BUG: Diagnostic contains: (long) x >>> 32
result += x >>> 32 ;
// BUG: Diagnostic contains: (long) x >> 40
result += x >> 40 ;
// BUG: Diagnostic contains: (long) (x & 255) >> 40
result += ( x & 255 ) >> 40 ;
// BUG: Diagnostic contains: 1L << 48
result += 1 << 48 ;
// BUG: Diagnostic contains: x >> 4
result += x >> 100 ;
// BUG: Diagnostic contains: x >> 31
result += x >> - 1 ;
byte b = 0 ;
char c = 'a' ;
// BUG: Diagnostic contains: (long) b >> 32
result += b >> 32 ;
// BUG: Diagnostic contains: (long) b << 32
result += b << 32 ;
// BUG: Diagnostic contains: (long) c >> 32
result += c >> 32 ;
// BUG: Diagnostic contains: (long) c >>> 32
result += c >>> 32 ;
}
}

Negative examples
BadShiftAmountNegativeCases.java

/*
* Copyright 2013 Google Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com . google . errorprone . bugpatterns . testdata ;
/**
* @author Bill Pugh (bill.pugh@gmail.com)
*/
public class BadShiftAmountNegativeCases {
public void foo () {
int x = 0 ;
long result = 0 ;
result += ( long ) x >> 3 ;
result += x << 3 ;
result += x >>> 3 ;
result += ( long ) ( x & 0xff ) >> 40 ;
}
}