Apex class that interprets JSON values and allows for easy traversal and type casting.

JSONValue.cls

public class JSONValue {
    private Object objValue;
    
    public JSONValue(String serialized) {
        this.objValue = JSON.deserializeUntyped(serialized);
    }

    private JSONValue(Object unserialized) {
        this.objValue = unserialized;
    }

    public JSONValue get(Object[] keys) {
        JSONValue level = this;
        for (Object key : keys) {
            level = level.get(key);
        }
        return level;
    }

    public JSONValue get(Object key) {
        return new JSONValue(this.getAsObject(key));
    }

    public Decimal getAsDecimal(Object key) {
        Object objValue = getAsObject(key);
        return objValue instanceof String
            ? Decimal.valueOf((String)objValue)
            : Decimal.valueOf((Double)objValue);
    }

    public Boolean getAsBoolean(Object key) {
        return (Boolean)getAsObject(key);
    }

    public Double getAsDouble(Object key) {
        return (Double)getAsObject(key);
    }

    public Integer getAsInteger(Object key) {
        return (Integer)getAsObject(key);
    }

    public String getAsString(Object key) {
        return (String)getAsObject(key);
    }

    public Object getAsObject(Object keys) {
        if (keys instanceof Object[]) {
            Object level = this.objValue;
            for (Object key : (Object[])keys) {
                level = getAsObject(level, key);
            }
            return level;
        }
        return getAsObject(this.objValue, keys);
    }

    private static Object getAsObject(Object value, Object key) {
        if (value instanceof Map<String,Object>) {
            if (key instanceof String) {
                Map<String,Object> mapValue = (Map<String,Object>)value;
                String strKey = (String)key;
                if (mapValue.containsKey(strKey)) {
                    return mapValue.get(strKey);
                }
            }
        }
        else if (value instanceof Object[]) {
            if (key instanceof Integer) {
                Object[] arrayValue = (Object[])value;
                Integer intKey = (Integer)key;
                Integer count = arrayValue.size();
                if (intKey < 0) intKey += count;
                if (0 <= intKey && intKey <= count) {
                    return arrayValue.get(intKey);
                }
            }
        }
        else {
            throw new TypeException('You can only use getter on an array or a map.');
        }
        throw new TypeException('The specified key could not be found.');
    }

    public Integer asInteger() {
        return (Integer)this.objValue;
    }

    public String asString() {
        return (String)this.objValue;
    }

    public Boolean asBoolean() {
        return (Boolean)this.objValue;
    }

    public Double asDouble() {
        return (Double)this.objValue;
    }

    public Object asObject() {
        return this.objValue;
    }

    public Decimal asDecimal() {
        return this.objValue instanceof String
            ? Decimal.valueOf((String)this.objValue)
            : Decimal.valueOf((Double)this.objValue);
    }
}

~example-runs.cls

String strJSON = '{"people":[{"firstName": "John","lastName":"Smith","attributes":{"isHuman":true,"isWeak":false}}]}';
JSONValue jv = new JSONValue(strJSON);
Assert.areEqual('John', jv.get('people').get(-1).get('firstName').asString());
Assert.areEqual('Smith', jv.get(new Object[]{'people', 0, 'lastName'}).asString());
Assert.areEqual(true, jv.get('people').get(0).get('attributes').getAsBoolean('isHuman'));
Assert.areEqual(false, jv.getAsBoolean(new Object[]{'people', -1, 'attributes', 'isWeak'}));