code: public Signer build() {
          Util.notNull(signingService, "KSI signing service");
          if (defaultHashAlgorithm == null) {
              this.defaultHashAlgorithm = HashAlgorithm.SHA2_256;
          }
          defaultHashAlgorithm.checkExpiration();
          if (policy == null) {
              this.policy = ContextAwarePolicyAdapter.createInternalPolicy();
          }
          KSISignatureComponentFactory signatureComponentFactory = new InMemoryKsiSignatureComponentFactory();
          KSISignatureFactory uniSignatureFactory = new InMemoryKsiSignatureFactory(policy, signatureComponentFactory);
          return new SignerImpl(signingService, uniSignatureFactory, defaultHashAlgorithm);
      }
docs: Builds and returns the {@link Signer} instance. If signing client isn't configured, {@link NullPointerException} is thrown.