Java – Convert negative binary to Integer

Review the following Java example to convert a negative integer in binary string back to an integer type.

  String binary = Integer.toBinaryString(-1);   // convert -1 to binary
                                                // 11111111 11111111 11111111 11111111 (two's complement)
  int number = Integer.parseInt(binary, 2);     // convert negative binary back to integer
  System.out.println(number);                   // output ??

The result is NumberFormatException!

Exception in thread "main" java.lang.NumberFormatException: For input string: "11111111111111111111111111111111" under radix 2
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:68)
	at java.base/java.lang.Integer.parseInt(Integer.java:652)
	at com.mkyong.crypto.bytes.Byte2.main(Byte2.java:17)

The NumberFormatException is due to overflow, the string 11111111111111111111111111111111 (length of 32) are unable to fit into the 32 bits int type?

1. Integer Overflow?

In Java, the int is a 32 bits two’s complement integer, the first leftmost bit or most significant bit is denote the sign of the numbers, 0 is positive, 1 is negative.

{1}1111111 11111111 11111111 11111111 = {1} = most significant bit, denote positive or negative

Since the leftmost bit is reserved for the sign of the numbers, the Java int 32 bits only has values from -2^31 (-2,147,483,648) to 2^31-1 (2,147,483,647).

  System.out.println(Integer.MIN_VALUE);  // -2,147,483,648 , -2^31
  System.out.println(Integer.MAX_VALUE);  // 2,147,483,647 , 2^31-1

Review the binary string again, that is a full 32 bits, 2^32-1 (4,294,967,295). The int type is unable to handle such a large number, and Java doesn’t support unsigned int.

11111111 11111111 11111111 11111111

2. Long

To solve it, we can use long, 64 bits two’s complement integer, has values from -2^63 to 2^63-1.

  String binary = Integer.toBinaryString(-1);

  //int number = Integer.parseInt(binary, 2);

  long l = Long.parseLong(binary, 2);                 // 11111111 11111111 11111111 11111111
  int number = (int) l;

  System.out.println(number);                         // -1

3. Java 8

For Java 8, it introduced new APIs to support unsigned operations, try Integer.parseUnsignedInt().

  String binary = Integer.toBinaryString(-1);

  //int number = Integer.parseInt(binary, 2);
  int number = Integer.parseUnsignedInt(binary, 2);

  System.out.println(number);                         // -1

References

• Java Primitive Data Types
• Java Sign Extension
• Wikipedia – two’s complement