1. A computer implemented method executed for realizing a runtime system for a class-based object-oriented programming language, the method comprising: preparing a first class that is an existing class in the object-oriented programming language, and a second class that is a class that includes a member that is accessible from outside of the first class and that is a class which is specialized for a specific use, wherein the first class comprises a java.math.BigInteger class; performing processing using the second class in accordance with a predetermined instruction in software that realizes the runtime system, wherein performing processing using the second class further comprises storing a bit string having a bit length less than 64 such that the second class stores a 63 rd bit to a 32 nd bit in a first 32-bit field of the second class, and further such that the second class stores a 31 st bit to a 0 th bit in a second 32-bit field of the second class, and further such that the second class includes a third 32-bit field containing the bit length of the bit string and a length until a first non-zero bit appears in the bit string, and further such that the second class includes a fourth 32-bit field storing a hashcode field, a fifth 32-bit field storing a class pointer field, and a sixth 32-bit field storing a digits field; and performing processing using the first class in accordance with an instruction to check an identity of the second class in a user program product that is executed by the runtime system.