BadShiftAmount
Shift by an amount that is out of range

Severity
ERROR
Has Fix?
REQUIRES_HUMAN_ATTENTION

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 the suppression annotation @SuppressWarnings("BadShiftAmount") to the enclosing element.


Positive examples

BadShiftAmountPositiveCases.java

/*
* Copyright 2013 The Error Prone Authors.
*
* 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 The Error Prone Authors.
*
* 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;
}
}