Methods

The simplest mechanism for encapsulating implementation details is a method.

The Math.sin function that comes with Java has a definition that looks something like this.¹

double sin(double x) {
    double[] y = new double[2];
    double z = 0.0;
    int n, ix;

    // High word of x.
    ix = __HI(x);

    // |x| ~< pi/4
    ix &= EXP_SIGNIF_BITS;
    if (ix <= 0x3fe9_21fb) {
        return __kernel_sin(x, z, 0);
    } else if (ix >= EXP_BITS) {  // sin(Inf or NaN) is NaN
         return x - x;
    } else { // argument reduction needed
        n = RemPio2.__ieee754_rem_pio2(x, y);
        // ...
    }
}

People who write programs that depend on the sin function do not generally understand how this works. I have a math minor² and I certainly don't.

All they need to know to use sin effectively is what it does, not how it does it. In this way methods are one way to provide encapsulation. You reduce a potentially complicated body of code to inputs required and outputs produced.³