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Web Automation for Java
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package com.ui4j.bytebuddy;

import com.ui4j.bytebuddy.jar.asm.Opcodes;

/**
 * A wrapper object for representing a validated class file version in the format that is specified by the
 * JVMS.
 */
public class ClassFileVersion implements Comparable {

    /**
     * The class file version of Java 1.
     */
    public static final ClassFileVersion JAVA_V1 = new ClassFileVersion(Opcodes.V1_1);
    /**
     * The class file version of Java 2.
     */
    public static final ClassFileVersion JAVA_V2 = new ClassFileVersion(Opcodes.V1_2);
    /**
     * The class file version of Java 3.
     */
    public static final ClassFileVersion JAVA_V3 = new ClassFileVersion(Opcodes.V1_3);
    /**
     * The class file version of Java 4.
     */
    public static final ClassFileVersion JAVA_V4 = new ClassFileVersion(Opcodes.V1_4);
    /**
     * The class file version of Java 5.
     */
    public static final ClassFileVersion JAVA_V5 = new ClassFileVersion(Opcodes.V1_5);
    /**
     * The class file version of Java 6.
     */
    public static final ClassFileVersion JAVA_V6 = new ClassFileVersion(Opcodes.V1_6);
    /**
     * The class file version of Java 7.
     */
    public static final ClassFileVersion JAVA_V7 = new ClassFileVersion(Opcodes.V1_7);
    /**
     * The class file version of Java 8.
     */
    public static final ClassFileVersion JAVA_V8 = new ClassFileVersion(Opcodes.V1_8);

    /**
     * The system property for this JVM's Java version.
     */
    private static final String JAVA_VERSION_PROPERTY = "java.version";
    /**
     * The version number that is represented by this class file version instance.
     */
    private final int versionNumber;

    /**
     * Creates a wrapper for a given minor-major release of the Java class file file and validates the
     * integrity of the version number.
     *
     * @param versionNumber The minor-major release number.
     */
    public ClassFileVersion(int versionNumber) {
        this.versionNumber = validateVersionNumber(versionNumber);
    }

    /**
     * Creates a class file version for a given major release of Java. Currently, all versions reaching from
     * Java 1 to Java 8 are supported.
     *
     * @param javaVersion The Java version.
     * @return A wrapper for the given Java class file version.
     */
    public static ClassFileVersion forKnownJavaVersion(int javaVersion) {
        switch (javaVersion) {
            case 1:
                return JAVA_V1;
            case 2:
                return JAVA_V2;
            case 3:
                return JAVA_V3;
            case 4:
                return JAVA_V4;
            case 5:
                return JAVA_V5;
            case 6:
                return JAVA_V6;
            case 7:
                return JAVA_V7;
            case 8:
                return JAVA_V8;
            default:
                throw new IllegalArgumentException("Unknown Java version: " + javaVersion);
        }
    }

    /**
     * Finds the highest class file version that is compatible to the current JVM version by parsing the java.version
     * property which is provided by {@link java.lang.System#getProperty(String)}.
     *
     * @return The currently running Java process's class file version.
     */
    public static ClassFileVersion forCurrentJavaVersion() {
        String versionString = System.getProperty(JAVA_VERSION_PROPERTY);
        int[] versionIndex = {-1, 0, 0};
        for (int i = 1; i < 3; i++) {
            versionIndex[i] = versionString.indexOf('.', versionIndex[i - 1] + 1);
            if (versionIndex[i] == -1) {
                throw new IllegalStateException("This JVM's version string does not seem to be valid: " + versionString);
            }
        }
        return ClassFileVersion.forKnownJavaVersion(Integer.parseInt(versionString.substring(versionIndex[1] + 1, versionIndex[2])));
    }

    /**
     * Validates the version number.
     *
     * @param versionNumber The parsed version number.
     * @return The same number.
     */
    private static int validateVersionNumber(int versionNumber) {
        if (!(versionNumber > 0)) {
            throw new IllegalArgumentException("Class version " + versionNumber + " is not valid");
        }
        return versionNumber;
    }

    /**
     * Returns the minor-major release number of this class file version.
     *
     * @return The minor-major release number of this class file version.
     */
    public int getVersionNumber() {
        return versionNumber;
    }

    /**
     * Checks if this class file version supports interface default methods. Such methods were introduced by Java 8.
     *
     * @return {@code true} if this class file version supports interface default methods.
     */
    public boolean isSupportsDefaultMethods() {
        return versionNumber > Opcodes.V1_7 && versionNumber != Opcodes.V1_1;
    }

    @Override
    public int compareTo(ClassFileVersion other) {
        return other.versionNumber == versionNumber
                ? 0 : versionNumber == Opcodes.V1_1
                ? -1 : other.versionNumber == Opcodes.V1_1
                ? 1 : versionNumber < other.versionNumber
                ? -1 : 1;
    }

    @Override
    public boolean equals(Object other) {
        return this == other || !(other == null || getClass() != other.getClass())
                && versionNumber == ((ClassFileVersion) other).versionNumber;
    }

    @Override
    public int hashCode() {
        return versionNumber;
    }

    @Override
    public String toString() {
        return "ClassFileVersion{versionNumber=" + versionNumber + '}';
    }
}